Power Converter Circuit with AC Output

ABSTRACT

A power converter circuit includes output terminals configured to receive an external voltage. A series circuit with a number of converter units is connected between the output terminals of the power converter circuit. Each converter unit includes input terminals configured to be coupled to a DC power source and output terminals configured to provide an output current. At least one converter unit of the converter units includes a signal generator configured to receive a synchronization signal and to generate a continuous synchronization signal from the synchronization signal. The power converter circuit can be operated in a normal operation mode. In the normal operation mode, the at least one converter unit is configured to regulate generation of the output current such that a frequency and/or a phase of the output current are dependent on the continuous synchronization signal.

This is a continuation-in-part application of U.S. application Ser. No.13/352,202, which was filed on Jan. 17, 2012 and is incorporated hereinby reference.

TECHNICAL FIELD

Embodiments of the present invention relate to a power convertercircuit, a power supply system with a power converter circuit, and amethod for operating a power converter circuit.

BACKGROUND

With an increasing interest in sustainable energy production there is afocus on using photovoltaic modules for producing electric power.Photovoltaic (PV) modules include a plurality of photovoltaic (PV)cells, that are also known as solar cells. Since the output voltage ofone cell is relatively low, a PV module usually includes a string with aplurality of series connected solar cells, such as between 50 to 100cells connected in series, or even several such strings connected inparallel.

A PV module provides a DC supply voltage, while power grids, such asnational power grids, have an AC supply voltage. In order to supply theenergy provided by a PV module to the power grid it is, therefore,necessary to convert the DC voltage of the PV module into an AC voltagethat is consistent with the AC supply voltage of the power grid.

A first approach for converting the PV module DC voltage into a powergrid AC voltage includes connecting several PV modules in series so asto obtain a DC voltage that is higher than the peak voltage of the powergrid AC voltage, and converting the DC voltage into the AC voltage usinga DC/AC converter. The amplitude of the DC voltage is typically between200V and 1000V. High DC voltages, however, are critical in terms of theoccurrence of electric arcs.

According to a second approach, a plurality of DC/AC converters areprovided, where each of these converters is connected to a PV module.The individual converters have their AC voltage outputs connected inparallel and each of these converters generates an AC voltage that isconsistent with the power grid AC supply voltage from the DC voltageprovided by the string of solar cells. The DC voltage provided by one PVmodule usually has an amplitude in the range of between 20V and 100V,depending on the number of cells that are connected in series within onemodule and depending on the technology used to implement the solarcells, while the peak voltage of the power grid AC voltage is about 155Vor 325V, depending on the country. However, due to the large differencebetween input and output voltages these converters have a disadvantagein terms of efficiency.

According to a further approach, several DC/AC converters are connectedin series, where each of these converters receives a DC supply voltagefrom a PV module. In this system a central control unit is employed tosynchronize the individual DC/AC converters in a multi-level switchingpattern. This system requires constant synchronized control of allindividual units.

There is, therefore, need for a power converter circuit that is capableof efficiently transforming relatively low DC supply voltages into an ACsupply voltage that is consistent with a power grid voltage.

SUMMARY OF THE INVENTION

A first aspect relates to a power converter circuit. The power convertercircuit includes output terminals configured to receive an externalvoltage, at least one series circuit with at least two converter unitsconnected between the output terminals of the power converter circuit,each converter unit including input terminals configured to be coupledto a DC power source, output terminals for providing an output current,and a signal generator, and at least one converter unit including asignal generator configured to receive a synchronization signal andconfigured to generate a continuous synchronization signal from thesynchronization signal. The power converter can be operated in a normaloperation mode, wherein in the normal mode, the at least one converterunit is configured to regulate a generation of the output current suchthat at least one of a frequency and a phase of the output current aredependent on the continuous synchronization signal.

A second aspect relates to a power supply system. The power supplysystem includes output terminals configured to receive an externalvoltage, at least one series circuit with at least two converter unitsconnected between the output terminals of the power converter circuit,each converter unit including input terminals and output terminals forproviding an output current, and at least one converter unit including asignal generator configured to receive a synchronization signal andconfigured to generate a continuous synchronization signal from thesynchronization signal. The power supply system further includes atleast two DC power sources, with each DC power source coupled to theinput terminals of one converter unit. The power supply system can beoperated in a normal operation mode, wherein in the normal mode, the atleast one converter unit is configured to regulate a generation of theoutput current such that at least one of a frequency and a phase of theoutput current are dependent on the continuous synchronization signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples will now be explained with reference to the drawings. Thedrawings serve to illustrate the basic principle, so that only aspectsnecessary for understanding the basic principle are illustrated. Thedrawings are not to scale. In the drawings the same reference charactersdenote like signals and circuit components.

FIG. 1 schematically illustrates a power converter circuit including aplurality of DC/AC converter units connected in series and a voltagemeasurement circuit;

FIG. 2 which includes FIGS. 2A-2C illustrates different embodiments ofphotovoltaic arrays, each including at least one solar cell;

FIG. 3 schematically illustrates a power converter circuit including aplurality of DC/AC converter units connected in series and a voltagemeasurement circuit including a plurality of measurement units connectedin series;

FIG. 4 which includes FIGS. 4A-4D illustrates different embodiments ofmeasurement units;

FIG. 5 shows a block diagram illustrating a first embodiment of oneDC/AC converter unit, including a DC/AC converter and a control circuit;

FIG. 6 illustrates an embodiment of the DC/AC converter of FIG. 5 indetail;

FIG. 7 which includes FIGS. 7A to 7C illustrates different embodimentsof switches that may be used in the DC/AC converter of FIG. 6;

FIG. 8 illustrates a first embodiment of the control circuit of oneDC/AC converter unit;

FIG. 9 illustrates a first branch of the control circuit of FIG. 8 indetail;

FIG. 10 illustrates a second embodiment of the control circuit of oneDC/AC converter unit;

FIG. 11 shows a block diagram illustrating a second embodiment of oneconverter unit, including a DC/DC converter, a maximum power pointtracker, a DC/AC converter, and a control circuit;

FIG. 12 illustrates an embodiment of the DC/DC converter implemented asa boost converter;

FIG. 13 schematically illustrates a control circuit of the DC/DCconverter of FIG. 12;

FIG. 14 illustrates an embodiment of the DC/DC converter implemented asa buck converter;

FIG. 15 illustrates a further embodiment of the control circuit of oneDC/AC converter;

FIG. 16 illustrates an embodiment of the DC/DC converter implementedwith two interleaved boost converter stages;

FIG. 17 illustrates a first embodiment of a control circuit for theDC/DC converter of FIG. 16;

FIG. 18 illustrates a second embodiment of a control circuit for theDC/DC converter of FIG. 16;

FIG. 19 shows a block diagram illustrating a further embodiment of oneDC/AC converter unit including a buck converter and an unfolding bridge;

FIG. 20 shows timing diagrams illustrating the operating principle ofthe DC/AC converter unit of FIG. 19;

FIG. 21 illustrates a first embodiment of a controller implemented inthe DC/AC converter unit of FIG. 19;

FIG. 22 illustrates a second embodiment of a controller implemented inthe DC/AC converter unit of FIG. 19;

FIG. 23 illustrates an embodiment of a power converter circuit having aplurality of converter units organized in two series circuits beingconnected in parallel.

FIG. 24 illustrates a further embodiment of the synchronization circuit.

FIG. 25 illustrates an embodiment of a transmission circuit in thesynchronization circuit of FIG. 24.

FIG. 26 illustrates a further embodiment of one converter unit.

FIG. 27 illustrates a first embodiment of a signal generator in theconverter unit of FIG. 26.

FIG. 28 illustrates timing diagrams of signals occurring in the signalgenerator of FIG. 27.

FIG. 29 illustrates a first embodiment of a signal generator in theconverter unit of FIG. 26.

FIG. 30 schematically illustrates two possible operation modes of thepower converter circuit.

FIG. 31 illustrates an embodiment of a power converter circuit includingan operation mode controller.

FIG. 32 illustrates an embodiment of a converter unit including anoperation mode unit.

FIG. 33 illustrates a first embodiment of a transfer from a firstoperation mode to a second operation mode.

FIG. 34 illustrates a second embodiment of a transfer from a firstoperation mode to a second operation mode.

FIG. 35 illustrates a further embodiment of a power converter circuit.

FIG. 36 illustrates an embodiment of a converter unit implemented in thepower converter circuit of FIG. 35.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In the following detailed description, reference is made to theaccompanying drawings, which form a part thereof, and in which is shownby way of illustration specific embodiments in which the invention maybe practiced. In this regard, directional terminology, such as “top,”“bottom,” “front,” “back,” “leading,” “trailing,” etc., is used withreference to the orientation of the figures being described. Becausecomponents of embodiments can be positioned in a number of differentorientations, the directional terminology is used for purposes ofillustration and is in no way limiting. It is to be understood thatother embodiments may be utilized and structural or logical changes maybe made without departing from the scope of the present invention. Thefollowing detailed description, therefore, is not to be taken in alimiting sense, and the scope of the present invention is defined by theappended claims. It is to be understood that the features of the variousexemplary embodiments described herein may be combined with each other,unless specifically noted otherwise.

In the following, embodiments of the present invention will be explainedin a specific context, namely in the context of converting electricalpower or electrical voltages provided by a plurality of photovoltaicarrays into an AC voltage, specifically an AC power grid supply voltage.However, this is only an example, embodiments of the invention may beemployed in a wide range of applications in which a conversion of DCvoltages into an AC voltage is required. Any type of DC power source maybe used instead of a photovoltaic array, such as a fuel cell. It is evenpossible, to employ DC power source of different types, such asphotovoltaic arrays and fuel cells, in one application.

FIG. 1 illustrates a first embodiment of a power converter circuit(power inverter circuit) 4 for converting a plurality of n (at leasttwo) DC input voltages V3 ₁, V3 ₂, V3 _(n) into one AC output voltagev1. It should be noted in this connection that throughout the drawingsDC voltages and DC currents will be denoted using capital letters “V”and “I”, while AC voltages and AC currents will be denoted usinglowercase letters “v” and “i”. The power converter circuit includes aplurality of n (at least two) converter units 2 ₁, 2 ₂, 2 _(n), withn≧2. Each of these converter units includes input terminals 21 ₁, 22 ₁;21 ₂, 22 ₂; and 21 _(n), 22 _(n) that are configured to be coupled to aDC power source 3 ₁, 3 ₂, 3 _(n). In FIG. 1, besides the power convertercircuit 1 with the converter units 2 ₁, 2 ₂, 2 _(n) DC power sources 3₁, 3 ₂, 3 _(n) are also illustrated. These DC power sources 3 ₁, 3 ₂, 3_(n) together with the power converter circuit 1 form an AC power supplysystem or an AC current supply system. The DC power sources 3 ₁, 3 ₂, 3_(n) are implemented as photovoltaic (PV) modules in the embodimentillustrated in FIG. 1. However employing PV modules as DC power sourcesis only an example. Any other type of DC power source, such as a powersource including fuel cells, may be used as well. It is even possible toemploy different types of DC power sources in one power supply system.

Each of the converter units 2 ₁, 2 ₂, 2 _(n) further includes outputterminals 23 ₁, 24 ₁; 23 ₂, 24 ₂; and 23 _(n), 24 _(n). The converterunits 2 ₁, 2 ₂, 2 _(n) are connected in series (in cascade) betweenoutput terminals 11, 12 of the power converter circuit 1. For this, afirst converter unit 2 ₁ has a first output terminal 23 ₁ coupled to afirst output terminal 11 of the power converter circuit 1 and a lastconverter unit 2 _(n) in the cascade has a second output terminal 24_(n) coupled to a second output terminal 12 of the power convertercircuit 1. Further, each of the first output terminals (other thanoutput terminal 23 ₁) are connected to one second output terminal (otherthan output terminal 24 _(n)) of another converter unit.

The output terminals 11, 12 of the power converter circuit 1 areconfigured to receive a voltage v1. For example, the output terminals11, 12 are configured to be connected to a power grid, so that theexternal voltage v1 corresponds to a grid voltage or, more specifically,corresponds to one phase of the power grid. In FIG. 1, the power grid isrepresented by a voltage source 100 and a load Z connected in parallelwith the power source 100. The voltage source 100 of the power gridrepresents a plurality of AC voltage sources in the power grid, and loadZ represents a plurality of loads connected to the power sources in thepower grid. The power grid defines the AC voltage v1 between the outputterminal. Since this voltage v1 is defined by an external source, suchas the power grid, this voltage will be referred to as external ACvoltage v1 in the following.

Each of the converter units 2 ₁, 2 ₂, 2 _(n) provides an AC outputvoltage v2 ₁, v2 ₂, v2 _(n) between its output terminals 23 ₁, 24 ₁, 23₂, 24 ₂, 23 _(n), 24 _(n). By having the converter units 2 ₁, 2 ₂, 2_(n) connected in series, the sum of the individual AC output voltagesv2 ₁, v2 ₂, v2 _(n) of the converter units 2 ₁, 2 ₂, 2 _(n) correspondsto the external voltage v1 when the power converter circuit 1 is in thesteady state, that is,

$\begin{matrix}{{v\; 1} = {\sum\limits_{i = 1}^{n}\; {v\; {2_{i}.}}}} & (1)\end{matrix}$

Each power converter unit 2 ₁, 2 ₂, 2 _(n) further includes an outputcapacitance C₁, C₂, C_(n) connected between the individual outputterminals 23 ₁, 24 ₁, 23 ₂, 24 ₂, 23 _(n), 24 _(n) and provides anoutput current i1 ₁, i1 ₂, i1 _(n). The output current of one converterunit 2 ₁, 2 ₂, 2 _(n) is the current received at a circuit node commonto the output capacitance C₁, C₂, C_(n) and one of the output terminals.For example, in the first converter unit 2 ₁, the output current of theconverter unit 2 ₁ is the current flowing into the circuit node at whichthe output capacitor C₁ is connected to the first output terminal 23 ₁.The current flowing from the first output terminal 23 ₁ of the firstconverter unit 2 ₁ will be referred to as converter circuit outputcurrent i_(OUT). This current corresponds to the current flowing betweenthe individual converter units 2 ₁-2 _(n). The output capacitances C₁,C₂, C_(n) are part of the individual converter units 2 ₁, 2 ₂, 2 _(n)and can be implemented in many different ways as will be explained withreference to several examples herein below.

In the steady state, the AC output currents i1 ₁, i1 ₂, i1 _(n) or, moreprecisely, the rms values of the AC output currents i1 ₁, i1 ₂, i1 _(n),correspond to the power converter circuit output current i_(OUT) or therms value of the output current i_(OUT), respectively, so that there isvery little to no rms current into the output capacitors C₁-C_(n).However, there can be situations in which the output currents i1 ₁, i1₂, i1 _(n) of the individual converter units 2 ₁, 2 ₂, 2 _(n) change andin which the output currents i1 ₁, i1 ₂, i1 _(n) are mutually differentuntil the system has settled at new (equal) output currents i1 ₁, i1 ₂,i1 _(n). This is explained in further detail below.

The power converter circuit 1 further includes a synchronization circuit10 connected between the output terminals 11, 12 of the power convertercircuit 1. The synchronization circuit 10 is configured to provide atleast one synchronization signal S_(v1). According to one embodiment,the synchronization signal is an AC signal having a phase and afrequency that are dependent on the phase and the frequency,respectively, of the external AC voltage v1.

The individual converter units 2 ₁, 2 ₂, 2 _(n) are each configured toreceive one synchronization signal S_(v1). In the embodiment illustratedin FIG. 1, the individual converter units 2 ₁, 2 ₂, 2 _(n) receive thesame synchronization signal S_(v1). However, this is only an example. Itis also possible to generate one synchronization signal for each of theconverter units 2 ₁, 2 ₂, 2 _(n). An embodiment is explained withreference to FIG. 3 herein below. The at least one synchronizationsignal S_(v1) can be transmitted to the individual converter units 2 ₁,2 ₂, 2 _(n) in different ways. Referring to FIG. 1, a signaltransmission bus can be provided through which the at least onesynchronization signal S_(v1) is transmitted to the individual converterunits 2 ₁, 2 ₂, 2 _(n). According to a further embodiment (notillustrated in FIG. 1), there is a dedicated transmission path betweenthe voltage synchronization circuit 10 and each of the converter units 2₁, 2 ₂, 2 _(n). The signal transmission bus or the signal transmissionpaths can be implemented like conventional signal transmission buses orlike conventional signal transmission paths. The signal bus or thesignal paths may include level shifter or other means to transmit the atleast one synchronization signal from the synchronization circuit 10 tothe individual converter units 2 ₁, 2 ₂, 2 _(n) that (due to beingconnected in series) have different reference potentials or differentvoltage domains.

The individual converter units 2 ₁, 2 ₂, 2 _(n) each include at leastone internal control loop which will be explained in further detailherein below. The control loop of each converter unit 2 ₁, 2 ₂, 2 _(n)is configured to have each converter unit 2 ₁, 2 ₂, 2 _(n) generate thecorresponding output current i1 ₁, i1 ₂, i1 _(n) such that there is agiven phase difference between the phase as represented by the at leastone synchronization signal S_(v1), and the phase of its AC outputcurrent i1 ₁, i1 ₂, i1 _(n). According to one embodiment, thesynchronization signal S_(v1) is in phase with the external AC voltagev1 and the individual output currents i1 ₁, i1 ₂, i1 _(n) are generatedto be in phase with the synchronization signal S_(v1) and, therefore,the external AC voltage v1, so that the phase difference is zero.According to another embodiment, the phase difference is not zero. Whensetting the difference to a value other than zero, reactive power is fedinto the power grid. This may help to stabilize the external AC voltage,which is e.g. a power grid.

In FIG. 1, same features of the DC voltage sources 3 ₁, 3 ₂, 3 _(n) havethe same reference characters, where the reference characters of theindividual DC voltage sources 3 ₁, 3 ₂, 3 _(n) can be distinguished fromeach other by subscript indices “1”, “2”, “n”. Equivalently, samefeatures of the converter units 2 ₁, 2 ₂, 2 _(n) have the same referencecharacters that can be distinguished by subscript indices, “1” for thefirst converter unit 2 ₁, “2” for the second converter unit 2 ₂ and “n”for the n-th converter unit 2 _(n). In the following, when explanationsequivalently apply to each of the DC sources 2 ₁, 2 ₂, 2 _(n) or to eachof the converter units 2 ₁, 2 ₂, 2 _(n), reference characters will beused without indices. In the following, reference character 2, forexample, represents an arbitrary one of the converter units, referencecharacter 23 represents a first output terminal of an arbitrary one ofthe converter units, reference character i1 denotes the output currentof an arbitrary converter unit 2, reference character denotes the outputcapacitance C of an arbitrary converter unit 2, and so on.

The power converter of FIG. 1 includes n=3 converter units 2. However,having n=3 converter units is only an example. Any number of n converterunits 2, wherein n>1, can be connected in series to form the powerconverter circuit 1.

Besides the internal control loops of the converter units 2 the powerconverter circuit 1 does not require an outer control loop connected tothe individual converter units 2 and/or additional communication pathsbetween the individual converter units 2, when the power convertercircuit 1 is in the steady state. When the power converter circuit 1 isin the steady state the system can be defined by equation (1) and onefurther equation for each of the converter units 2:

v2_(RMS) ·i1_(RMS) =V3·I3   (2),

where v2rms denotes the RMS (route mean square) value of the outputvoltage v2 of one converter units 2, i1 _(RMS) denotes the RMS value ofthe output current i1 of one converter unit, V3 denotes the inputvoltage and I3 denotes the input current of the converter unit 2. Itshould be noted that (very low) losses may occur in each converter unit2. For the sake of simplicity, these losses are not considered inequation (2). In the steady state, the RMS values of the individualoutput currents i1 _(RMS) are equal and correspond to the rms value ofthe power converter circuit output current i_(OUT-RMS), that is:

i1_(RMS) =i1_(OUT-RMS)   (3)

Since equations (2) and (3) is valid for each of the individualconverter units, there are n equations, each of these equationsdescribing the relationship between the input power and the averageoutput power of each of the converter units 2, where the input power Pinis given as

Pin=V3·I3   (4),

and the output power Pout is given as

Pout=v2_(RMS) ·i1_(RMS)   (5).

The input power Pin of each of the individual converter units 2 and theinput voltage V3 and the input current I3, respectively, are externalparameters given by the individual DC power sources 3. The external ACvoltage v1 between the output terminals 11, 12 is defined by the powergrid.

Thus, there are n+1 variables in the power converter circuit 1, namelythe n output voltages v2 of the individual converter units 2 and the(equal) output currents i1. However, referring to equations (1) and (2)the system is defined by n+1 equations, so that each of the n+1variables is determined when the system is in its steady state. Besideshaving each of the converters 2 generate its AC output current i1 suchthat there is a given phase difference (such as zero) between the ACoutput current i1 and the external AC voltage no additional control orregulation mechanism is required. When the output currents i1 of theindividual converter units 2 are in phase with the external AC voltagev1 the real output power of each converter unit equals the apparentoutput power, so that the reactive output power is zero. The individualconverter units 2 control their output currents i1 dependent on thephase information as represented by the at least one synchronizationsignal S_(v1) and control their output current such that the input powerreceived at the input terminals 21, 22 equals the output power at theoutput terminals 23, 24.

The DC power sources 3 implemented as PV arrays are only schematicallyillustrated in FIG. 1. These PV arrays each include at least one solarcell. Some exemplary embodiments of PV arrays including at least onesolar cell are illustrated in FIGS. 2A to 2C. FIG. 2A illustrates afirst embodiment. In this embodiment, the PV array 3 includes only onesolar cell 31. Referring to a further embodiment illustrated in FIG. 2B,one PV array 3 includes a string of m solar cells 31, 3 m wherein m>1,connected in series. According to yet another embodiment illustrated inFIG. 2C, p strings of solar cells are connected in parallel, whereinp>1. Each of the strings includes m solar cells 31 ₁, 3 m ₁, 31 _(p), 3m _(p). However, the embodiments illustrated in FIGS. 2A to 2C are onlyexemplary. Many other solar cell arrangements may be used as well as aDC source 3.

FIG. 3 illustrates an embodiment of a power converter circuit thatincludes a synchronization circuit 10 implemented as a voltagemeasurement circuit with a plurality of measurement units 10 ₁, 10 ₂, 10_(n). The individual measurement units 10 ₁, 10 ₂, 10 _(n) are connectedin series between the output terminals 11, 12. For simplicity of theillustration, the output capacitances (C₁-C_(n) in FIG. 1) are notillustrated in FIG. 3. The plurality of measurement units 10 ₁, 10 ₂, 10_(n) forms a voltage divider, wherein a voltage drop v1 ₁, v1 ₂, v1 _(n)across each of the measurement units 10 ₁, 10 ₂, 10 _(n) is a functionof the external AC voltage v1 and includes an information on thefrequency and the phase of the external AC voltage v1. In thisembodiment, each converter unit 2 ₁, 2 ₂, 2 _(n) has a synchronizationinput with two input terminals 25 ₁, 26 ₁, 25 ₂, 26 ₂, 25 _(n), 26 _(n),and each converter unit 2 ₁, 2 ₂, 2 _(n) has the synchronization inputterminals coupled to one measurement unit 10 ₁, 10 ₂, 10 _(n), so as toreceive one measurement voltage v1 ₁, v1 ₂, v1 _(n) as thesynchronization signal.

In the embodiment illustrated in FIG. 3, the number of measurement units10 ₁, 10 ₂, 10 _(n) corresponds to the number of converter units 2 ₁, 2₂, 2 _(n), so that each measurement unit 10 ₁, 10 ₂, 10 _(n) isassociated with one converter unit 2 ₁, 2 ₂, 2 _(n). However, this isonly an example. According to a further embodiment (not illustrated) themeasurement voltage provided by one measurement unit is received by twoor more converter units.

The individual measurement units 10 ₁, 10 ₂, 10 _(n) can be implementedin many different ways. Some examples are explained with reference toFIGS. 4A to 4D below. In these FIGS. 4A to 4D, reference character 10,denotes an arbitrary one of the measurement units 10 ₁, 10 ₂, 10 _(n)illustrated in FIG. 3.

Referring to FIG. 4A, one measurement unit 10 i may include a resistor101 connected between the terminals of the measurement unit 10 i thatserve to connect the individual measurement units (10 ₁-10 _(n) in FIG.3) in series and that serve to couple the individual measurement unitsto the converter units (2 ₁-2 _(n) in FIG. 3). According to oneembodiment, the resistances of the resistors 101 in the individualmeasurement units 10 _(i) are equal or at least approximately equal. Inthis case, the absolute values of the measurement voltages v1 _(i)provided by the individual measurement units 10 _(i) are equal. In ameasurement circuit 10 that includes measurement units 10 _(i)implemented with a resistor 101, the individual measurement voltages v1_(i) are proportional to the output voltage v1.

In a measurement circuit 10 with measurement units 10 _(i) includingresistors, the individual measurement units 10 _(i) form a resistivevoltage divider. Referring to a further embodiment illustrated in FIG.4B, the individual measurement units 10 _(i) each include a capacitor102 instead of a resistor. In this case, the individual measurementunits 10 _(i) form a capacitive voltage divider between the outputterminals 11, 12.

Referring to FIG. 4C, that illustrates a further embodiment, eachmeasurement unit 10 i can be implemented with a parallel circuitincluding a resistor 101 and a capacitor 102.

Referring to FIG. 4D, that illustrates yet another embodiment of onemeasurement unit 10 _(i), each measurement unit or at least some of themeasurement units can be implemented with a voltage divider having afirst voltage divider 101 and a second voltage divider element 102.These voltage divider elements are implemented as resistors in theembodiment according to FIG. 4D. However, these voltage divider elements101, 102 could also be implemented as capacitors or as combinations withat least one resistor and at least one capacitor. In this embodiment,the measurement voltage v1 _(i) is not the voltage across themeasurement unit 10 _(i), but is the voltage across the first voltagedivider element 101, so that the measurement voltage v1 _(i) is afraction of the voltage across the measurement unit 10 _(i).

It should be noted that implementing the synchronization circuit 10 as avoltage measurement circuit that generates the synchronization signalS_(v1) to be in phase with the external AC voltage v1 is only anexample. Examples of other synchronization circuits are explained hereinfurther below.

FIG. 5 illustrates a first embodiment of a converter unit 2 forconverting the DC input voltage provided by one DC source (not shown inFIG. 3) into an AC output voltage v2. The converter unit 2 includes aDC/AC converter 4 connected between the input terminals 23, 22 and theoutput terminals 23, 24. The DC/AC converter receives the DC voltage V3provided by the DC power source as an input voltage and the DC supplycurrent I3 of the DC power source as an input current. The DC/ACconverter 4 further receives a reference signal S_(REF), which may be analternating signal having a frequency and a phase. The DC/AC converter 4is configured to generate the AC output current i1 dependent on thereference signal S_(REF) such that a frequency and a phase of the outputcurrent i1 correspond to a frequency and a phase, respectively, of thesynchronization signal S_(v1). The DC/AC converter 4 can be implementedlike a conventional DC/AC converter that is configured to generate anoutput current in phase with an alternating reference signal. Such DC/ACconverters are commonly known.

It should be noted that each of the DC/AC converter units 2 ₁, 2 ₂, 2_(n) controls its output current i1 to have a phase and frequency thatis dependent on the at least one synchronization signal S_(v1).

The reference signal S_(REF) is generated by a control circuit 5dependent on the synchronization signal S_(v1) and an output currentsignal S_(i1). The synchronization signal S_(v1) is either thesynchronization signal S_(v1) explained with reference to FIG. 1, one ofthe measurement voltages v1 _(i) explained with reference to FIG. 3, ora scaled version or a fraction thereof. The output current signal S_(i1)represents the output current i1, i.e. the output current signal S_(i1)is dependent on the output current i1. According to one embodiment, theoutput current signal S_(i1) is a scaled version of the output currenti1. The output current signal S_(i1) can be generated in a conventionalmanner from the output current i1 using a current measurement circuit(not illustrated). The output current signal S_(i1) is generated foreach of the converter units (2 ₁-2 _(n)) individually by detecting therespective output current of each converter unit. Referring to FIG. 5,the output current i1 of the illustrated converter unit 2 is the currentreceived at the circuit node common to the first output terminal 23 andthe output capacitance C.

The control circuit 5, which will also be referred to as controller inthe following, generates the reference signal S_(REF) dependent on thesynchronization signal S_(v1) and the output current signal S₁ such thatthe output current, when generated in correspondence with the referencesignal S_(REF), is in phase with the external AC voltage v1 or has agiven phase shift relative to the external AC voltage v1. It should benoted that, since the external AC voltage v1 and the output current i1are alternating signals, the synchronization signal S_(v1) and theoutput current signal S_(i1) are also alternating signals. In theconverter unit 2, the DC/AC converter 4 and the controller 5 are part ofa control loop that controls the output current i1 to be in phase withthe external AC voltage v1 or to have a given phase shift relativethereto.

Although a conventional DC/AC converter may be used in the converterunit 2 as the DC/AC converter 4 connected between the input terminals21, 22 and the output terminals 23, 24, one example of a DC/AC converter4 will be explained in detail with reference to FIG. 6, in order to easeunderstanding of embodiments of the invention.

The DC/AC converter 4 illustrated in FIG. 6 is a full-bridge (H4)converter with two half-bridge circuits each connected between the inputterminals 21, 22. Each of these half-bridge circuits includes twoswitches each having a load path and a control terminal. The load pathsof the two switches of one half-bridge circuit are connected in seriesbetween the input terminals 21, 22, where a first switch 42 ₁ and asecond switch 42 ₂ form the first half-bridge, and a third switch 42 ₃and a fourth switch 42 ₄ form the second half-bridge. Each of thehalf-bridges includes an output, where an output of the firsthalf-bridge is formed by a circuit node common to the load paths of thefirst and second switch 42 ₁, 42 ₂. An output of the second half-bridgeis formed by a circuit node common to the load paths of the third andfourth switches 42 ₂, 42 ₄. The output of the first half-bridge iscoupled to the first output terminal 23 of the converter unit 2 via afirst inductive element 44 ₁, such as a choke. The output terminal ofthe second half-bridge is coupled to the second output terminal 24 ofthe converter unit 2 via a second inductive element 44 ₂, such as achoke. According to a further embodiment (not illustrated) only one ofthe first and second inductive elements 44 ₁, 44 ₂ is employed. Theconverter 4 further includes an input capacitance 41, such as acapacitance, connected between the input terminals 21, 22, and theoutput capacitance C connected between the output terminals 23, 24.

Each of the switches 42 ₁, 42 ₂, 42 ₃, 42 ₄ receives a control signalS42 ₁, S42 ₂, S42 ₃, S42 ₄ at its control terminal. These controlsignals S42 ₁-S42 ₄ are provided by a drive circuit 45 dependent on thereference signals S_(REF) received from the controller 5. The drivesignal S42 ₁-S42 ₄ are pulse-width modulated (PWM) drive signalsconfigured to switch the corresponding switch 42 ₁-42 ₄ on and off. Itshould be noted that a switching frequency of the PWM signals S42 ₁-S42₄ is significantly higher than a frequency of the alternating referencesignal S_(REF). The reference signal S_(REF) may be a sinusoidal signalwith a frequency of 50 Hz or 60 Hz, depending on the country in whichthe power grid is implemented, while the switching frequency of theindividual switches 42 ₁-42 ₄ may be in the range of several kHz up toseveral 10 kHz, or even up to several 100 kHz. The drive circuit 45 isconfigured to individually adjust the duty cycle of each of the drivesignals S42 ₁-S42 ₄ between 0 and 1 in order to have the waveform of theoutput current i1 follow the waveform of the reference signal S_(REF).When the duty cycle of one drive signal is 0, the corresponding switchis permanently switched off, and when the duty cycle of one drive signalis 1, the corresponding switch is permanently switched on. The dutycycle of a drive signal is the relationship between the time period forwhich the drive signal switches the corresponding switch and theduration of one switching cycle. The duration of one switching cycle isthe reciprocal of the switching frequency.

Referring to what has been explained before, the output current i1 is anAC current with a positive half-cycle in which the output current ispositive, and with a negative half-cycle in which the output current i1is negative. The time behavior of the output current i1 is dependent onthe reference signal S_(REF) which also has positive and negativehalf-cycles.

Two possible operating principles of the converter 4 will briefly beexplained. First, it is assumed that a positive half-cycle of the outputcurrent i1 is to be generated. According to a first operating principle,which is known as bipolar switching or 2-level switching, the first andfourth switches 42 ₁, 42 ₄ are switched on and off synchronously, whilethe second and third switches 42 ₂, 42 ₃ are permanently switched off.During an on-phase of the first and fourth switches 42 ₁, 42 ₄ an outputcurrent i1 is forced through the choke(s) 44 ₁, 44 ₂ that is dependenton the voltage difference between the input voltage V3 across the inputcapacitance 41 and the output voltage v2, where the output voltage v2 isdefined by the power grid voltage v_(N). The switches 42 ₁-42 ₄ eachinclude a freewheeling element, such as a diode, that is alsoillustrated in FIG. 4. The freewheeling elements of the second and thirdswitches 42 ₂, 42 ₃ take the current flowing through the choke(s) 44 ₁,44 ₂ when the first and fourth switches 42 ₁, 42 ₄ are switched off. Inthis method, the amplitude of the output current i1 can be adjustedthrough the duty cycle of the synchronous switching operation of thefirst and fourth switches 42 ₁, 42 ₄. When the switching frequency ofthe switches 42 ₁, 42 ₄ is much higher than the desired frequency of theoutput current, amplitude, frequency and phase of the AC output currenti1 can be adjusted dependent on the reference signal S_(REF) through theduty cycle of the synchronous switching operation of the first andfourth switches 42 ₁, 42 ₄. During the negative half-cycle the secondand third switches 42 ₂, 42 ₃ are switched on and off synchronously,while the first and fourth switches 42 ₁, 42 ₄ are permanently off, sothat the body diodes of these first and fourth switches 42 ₁, 42 ₄ areconducting. Alternatively, the switches 42 ₁, 42 ₄ are switched (withshort dead times) when their body diodes are forward biased, so as to beoperated as synchronous rectifiers.

According to a second operating principle, which is known as phasechopping or 3-level switching, the first switch 42 ₁ is permanentlyswitched on during the positive half cycle of the output voltage v2, thesecond and third switches 42 ₂, 42 ₃ are permanently off, and the fourthswitch 42 ₄ is switched on and off in a clocked fashion. During anon-phase of the first and fourth switches 42 ₁, 42 ₄ an output currenti1 is forced through the choke(s) 44 ₁, 44 ₂ that is dependent onvoltage difference between the input voltage V3 across the inputcapacitance 41 and the output voltage v2, where the output voltage v2 isdefined by the power grid voltage v_(N). During an off-phase of thefourth switch 42 ₄ a freewheeling path is offered by the freewheelingelement of switch 42 ₃ and the switched-on first switch 42 ₁ thusenabling a zero volt state across the output chokes. In this method, theamplitude of the output current i1 can be adjusted through the dutycycle of the switching operation of the fourth switch 42 ₁, 42 ₄. Duringthe negative half-cycle the first and fourth switches 42 ₁, 42 ₄ arepermanently switched off, the second switch 42 ₂ is permanently switchedon, and the third switch 42 ₃ is switched on and off in a clockedfashion.

In order to control an instantaneous amplitude of the output current i1during the positive half-cycle, the drive circuit 45 varies the dutycycle of the at least one switch that is switched on and off in aclocked fashion. The duty cycle of the at least one clocked switch andthe duty cycle of its drive signal, respectively, is increased in orderto increase the amplitude of the output current i1 and is decreased inorder to decrease the amplitude of the output current i1. This dutycycle is dependent on the instantaneous amplitude of the referencesignal S_(REF).

The switches 42 ₁-42 ₄ may be implemented as conventional electronicswitches. Referring to FIG. 7A, which illustrates a first embodiment forimplementing the switches, the switches may be implemented as MOSFETs,specifically as n-type MOSFETs. Electronic switch 42 in FIG. 7Arepresents an arbitrary one of the switches 42 ₁-42 ₄. A MOSFET, such asthe n-type MOSFET illustrated in FIG. 7A has an integrated diode that isalso illustrated in FIG. 7A. This diode is known as body diode and mayact as a freewheeling element. A drain-source path, which is a pathbetween a drain terminal and a source terminal, forms a load path of aMOSFET, and a gate terminal forms a control terminal.

Referring to FIG. 7B, the switches 42 ₁-42 ₄ could also be implementedas IGBTs, where additionally a diode may be connected between acollector and an emitter terminal of the IGBT. This diode acts as afreewheeling element. In an IGBT, the load path runs between the emitterand the collector terminal, and the gate terminal forms a controlterminal.

According to a further embodiment, two of the four switches, such as thefirst and third transistors 421, 423 are implemented as SCR Thyristors,while the other two switches are implemented as MOSFET.

According to yet another embodiment, illustrated in FIG. 7C, theswitches 421-424 can be implemented as GaN-HEMTs (Gallium-Nitride HighElectron Mobility Transistors). Unlike a conventional (silicon orsilicon carbide) MOSFET a GaN-HEMT does not include an integrated bodydiode. In a GaN-HEMT, a current conduction in a reverse direction(corresponding to the forward direction of a body diode in aconventional MOSFET) can be obtained through a substrate biased turn-on.When implementing the switches in GaN technology, all switches of oneconverter unit can be implemented on a common semiconductor substrate.

FIG. 8 schematically illustrates an embodiment of the controller 5 thatgenerates the reference signal S_(REF) dependent on the synchronizationsignal S_(V1) and the output current signal S_(i1). FIG. 8 shows a blockdiagram of the controller 5 in order to illustrate its operatingprinciple. It should be noted that the block diagram illustrated in FIG.8 merely serves to illustrate the functionality of the controller 5rather than its implementation. The individual function blocks, thatwill be explained in further detail below, may be implemented using aconventional technology that is suitable to implement a controller.Specifically, the function blocks of the controller 5 may be implementedas analog circuits, digital circuits, or may be implemented usinghardware and software, such as a microcontroller on which a specificsoftware is running in order to implement the functionality of thecontroller 5.

Referring to FIG. 8, the controller 5 includes a phase locked loop (PLL)51 that provides a frequency and phase signal S_(ωt) representing thefrequency and the phase of the synchronization signal S_(v1).Specifically, S_(ωt) represents an instantaneous phase angle of the(sinusoidal) synchronization signal received at the input of the controlcircuit 5. Thus, signal S_(ωt) will also be referred to as phase anglesignal in the following. The PLL 51 receives the synchronization signalS_(v1), The frequency and phase signal S_(ωt) provided by the PLL 51 isreceived by a signal generator, such as a VCO, that generates asinusoidal signal S_(i1-REF) being in phase with the synchronizationsignal S_(v1) and forming a reference signal for the output current i1of the converter unit 2.

Referring to FIG. 8, the controller further receives the output currentsignal S_(i1) and calculates an error signal by subtracting the outputcurrent signal Si1 from the output current reference signal S_(i1-REF).The subtraction operation is performed by a subtractor receiving theoutput current measurement signal S_(i1-REF) and the output currentsignal S_(i1) at input terminals and providing the error signal at anoutput terminal. The error signal, which is also a sinusoidal signal isfiltered in a filter 53 connected downstream the subtractor 54. Thereference signal S_(REF) is a filtered version of the error signalavailable at the output of the filter 53. The filter is, e.g., aproportional (P) filter.

Optionally, a phase signal S_(φ) is added to the output signal of thePLL 51 before generating the sinusoidal reference signal S_(i1-REF). Inthis embodiment the reference signal S_(i1-REF) and, therefore, theoutput current i1, has a phase relative to the synchronization signalS_(v1), with the phase shift being defined by the phase signal S_(φ).

FIG. 9 illustrates an embodiment of the PLL 51 of FIG. 6. This PLLincludes a phase detector with a calculation unit 511 that calculatesthe sine or the cosine of the phase angle signal S_(ωt), and multiplier512 that receives the output signal from the calculation unit 511 andthe synchronization signal S_(v1). An error signal S_(ERROR) isavailable at the output of the multiplier 512. The error signalS_(ERROR) is received by a linear filter (LF) 514, such as, e.g., alinear proportional-integral (PI) filter. In the steady state, an outputsignal S_(ω) of the linear filter represents the frequency of thesynchronization signal S_(v1). An integrating circuit (a filter with anintegrating (I) characteristic) receives the output signal from thelinear filter, integrates the output signal of the linear filter 514 andprovides the frequency and phase signal (the phase angle signal) S_(ωt),from which the VCO (see 52 in FIG. 8) generates the reference signalS_(i1-REF). Integrating the output signal of the linear filter in thetime domain corresponds to a multiplication with 1/s in the frequencydomain.

FIG. 10 illustrates a further embodiment of the of the controller 5. Inthis embodiment, a second PLL 51′ receives the output current signalS_(i1) and calculates a further frequency and phase signal representingfrequency and phase of the output current signal S_(i1). The furtherfrequency and phase signal is subtracted from the frequency and phasesignal S_(ωt) representing frequency and phase of the synchronizationsignal S_(v1) (and, optionally, the phase shift S_(φ)) using asubtractor 54, so as to provide an error signal. The error signal isfiltered using a filter 53 and a signal generator 52, such as a VCO,receives the error signal and generates a sinusoidal reference signalwith frequency and phase defined by the filtered error signal. In thisembodiment, the filter 53 can be implemented as a P-filter or as aPI-filter.

FIG. 11 illustrates a further embodiment of one converter unit 2. Thisconverter unit besides the DC/AC converter 4 and the controller 5includes a DC/DC converter 6 connected between the input terminals 21,22, and the DC/AC converter 4. The DC/AC converter 4 may be implementedas explained with reference to FIGS. 6 to 10 with the difference thatthe DC/AC converter 4 of FIG. 11 receives a DC input voltage V6 from theDC/DC converter 6 instead of the input voltage V3 of the converter unit2. A capacitor 60 connected between the terminals 61, 62 may representan output capacitor of the DC/DC converter 6 or an input capacitor 4 ofthe DC/AC converter 4, or both. This capacitor 60 can be referred to asDC link capacitor.

The DC/DC converter 6 is configured to adjust the input voltage V3 orthe input current I3 to a voltage or current value, respectively, thatis dependent on a reference signal S_(REF-V3) received by the DC/DCconverter 6. For explanation purposes it is assumed that the DC/DCconverter 6 adjusts the input voltage V3 dependent on the referencesignal S_(REF-V3). Adjusting the input voltage V3 of the converter unit2 may help to operate the DC power source 3 connected to the inputterminals 21, 22, in an optimum operating point. This will be explainedin the following.

A solar cell and, therefore, a PV module including several solar cells,acts like a power generator providing a DC output voltage and a DCoutput current when it is exposed to sunlight. For a given light powerreceived by the PV array there is a range of output currents and a rangeof corresponding output voltages at which the PV array can be operated.However, there is only one output current and one corresponding outputvoltage at which the electric power provided by the PV array has itsmaximum. The output current and the output voltage at which the outputpower assumes its maximum define the maximum power point (MPP). The MPPvaries dependent on the light power received by the array and dependenton the temperature.

Referring to FIG. 11, the converter unit 2 further includes a maximumpower point tracker (MPPT) 7 that is configured to provide the referencesignal S_(REF-V3) such that DC/DC converter 6 adjusts the input voltagesuch that the DC source 3 is operated in its MPP. The MPPT 7 receives aninput current signal S_(I3) that represents the input current I3provided by the DC source 3 (illustrated in dashed lines in FIG. 9), andan input voltage signal S_(V3) that represents the input voltage V3provided by the DC source 3. From the input current signal SI3 and theinput voltage signal S_(V3) the MPPT 7 calculates the instantaneousinput power provided by the DC source 3. The input voltage signal S_(V3)can be obtained from the input voltage V3 in a conventional manner by,for example, using a voltage measurement circuit. Equivalently, theinput current signal S_(I3) can be obtained from the input current I3 ina conventional manner using, for example, a current measurement circuit.Those voltage measurement circuits and current measurement circuits arecommonly known and are not illustrated in FIG. 11.

The basic operating principle of the MPPT 7 in order to find the MPP isto vary the reference signal S_(REF-V3) within a given signal range andto determine the input power provided by the DC source 3 for each of theinput voltages V3 defined by the different reference signals S_(REF-V3).The MPPT 7 is further configured to detect the input voltage V3 forwhich the maximum input power has been obtained, and to finally set thereference signal S_(REF-V3) to that value for which the maximum inputpower has been detected.

Since the solar energy received by the PV array 3 may vary the MPPT 7 isfurther configured to check whether the DC source 3 is still operated inits maximum power point either regularly or when there is an indicationthat the maximum power point might have changed. An indication that themaximum power point might have changed is, for example, when the inputcurrent I3 represented by the input current signal S_(I3) changeswithout the reference signal S_(REF-V3) having changed. The regularcheck or the event-driven check of the MPPT 7 whether the DC source 3 isstill operated in its maximum power point, may include the samealgorithm that has been explained before for detecting the maximum powerpoint for the first time. Conventional algorithms for detecting themaximum power point that can be implemented in the MPPT 7 include, forexample, a “hill climbing algorithm” or a “perturb-and-observealgorithm”.

The DC/DC converter 6 can be implemented like a conventional DC/DCconverter. A first embodiment of a DC/DC converter 6 that can be used inthe converter unit 2 is illustrated in FIG. 12. The DC/DC converter 6illustrated in FIG. 12 is implemented as a boost converter. This type ofconverter includes a series circuit with an inductive storage element64, such as a choke, and a switch 65 between the input terminals of theDC/DC converter 6, where the input terminals of the DC/DC converter 6correspond to the input terminals 21, 22 of the converter unit 2.Further, a rectifier element 66, such as a diode, is connected between acircuit node common to the inductive storage element 64 and the switch65 and a first output terminal 61 of the DC/DC converter 6. A secondoutput terminal 62 of the DC/DC converter 6 is connected to the secondinput terminal 22. An output voltage V6 of the DC/DC converter isavailable between the output terminals 61, 62. Referring to FIG. 12, theDC/DC converter 6 may further include a first capacitive storage element63, such as a capacitor, between the input terminals 21, 22, and asecond capacitive storage element 68, such as a capacitor, between theoutput terminals 61, 62. The second capacitive storage element 68 actsas an energy storage that is necessary when generating the AC outputcurrent i1 from the DC voltage V6 available at the output of the DC/DCconverter 6.

The switch 65 can be implemented as a conventional electronic switch,such as a MOSFET or an IGBT. Further, the rectifying element 66 could beimplemented as a synchronous rectifier, which is a rectifier implementedusing an electronic switch, such as a MOSFET or an IGBT. According to afurther embodiment, the switch 65 is implemented as GaN-HEMT.

The DC/DC converter 6 further includes a control circuit (controller) 67for generating a drive signal S65 for the switch 65. This drive signalS65 is a pulse-width modulated (PWM) drive signal. The PWM controller 67is configured to adjust the duty cycle of this drive signal S65 suchthat the input voltage V3 corresponds to the desired input voltage asrepresented by the reference signal S_(REF-V3). For this, the controlcircuit 67 receives the reference signal S_(REF-V3) and the inputvoltage signal S_(V3) that represents the input voltage V3.

A first embodiment of the PWM control circuit 67 is illustrated in FIG.13 Like in FIG. 8 (which illustrates an embodiment of the controller 5)in FIG. 11 functional blocks of the controller 67 are illustrated. Thesefunctional blocks can be implemented as analog circuits, as digitalcircuits or can be implemented using hardware and software. Referring toFIG. 13, the control circuit 67 calculates an error signal S_(ERR) fromthe input voltage signal S_(V3) and the reference signal S_(REF-V3). Theerror signal S_(ERR) is calculated by either subtracting the inputvoltage signal V3 from the reference signal S_(REF-V3) (as illustrated)or by subtracting the reference signal S_(REF-V3) from the input voltagesignal S_(V3). The error signal S_(ERR) is provided by a subtractionelement 671 that receives the input voltage signal S_(V3) and thereference signal _(SREF-V3).

The error signal S_(ERR) is received by a filter 672 that generates aduty cycle signal S_(DC) from the error signal S_(ERR). The duty cyclesignal S_(DC) represents the duty cycle of the drive signal S65 providedby the control circuit 67. The filter 672 can be a conventional filterfor generating a duty cycle signal S_(DC) from an error signal S_(ERR)in a PWM controller of a DC/DC converter, such as a P-filter, aPI-filter, or a PID-filter.

A PWM driver 673 receives the duty cycle signal S_(DC) and a clocksignal CLK and generates the drive signal S65 as a PWM signal having aswitching frequency as defined by the clock signal CLK and a duty cycleas defined by the duty cycle signal S_(DC). This driver 673 can be aconventional PWM driver that is configured to generate a PWM drivesignal based on a clock signal and a duty cycle information. Suchdrivers are commonly known, so that no further information are requiredin this regard.

The basic control principle of the controller 67 of FIG. 12 will brieflybe explained. Assume that the input voltage V3 has been adjusted to agiven value represented by the reference signal S_(REF-V3) and that thereference signal S_(REF-V3) changes, so that the input voltage V3 has tobe re-adjusted. For explanation purposes it is assumed that the inputvoltage V3 is to be increased as defined by the reference signalS_(REF-V3). In this case the control circuit 67 reduces the duty cycleof the drive signal S65. Reducing the duty cycle of the drive signal S65results in a decreasing (average) input current I3, where decreasing theinput current I3, at a given power provided by the DC source 3 resultsin an increasing input voltage V3. Equivalently, the duty cycle isincreased when the input voltage V3 is to be decreased. An increase inthe duty cycle results in an increase of the input current I3.

The boost converter according to FIG. 12 does not only provide a load tothe DC source 3 in order to operate the DC source 3 in its maximum powerpoint. This boost converter also generates an output voltage V6 receivedby the DC/AC converter 4 (see FIG. 11) that his higher than the inputvoltage V3. Further, the boost converter is implemented such that theoutput voltage V6 is higher than a peak voltage of the output voltage v2of the DC/AC converter, but lower than a voltage blocking capability ofthe switches (see 42 ₁-42 ₄ in FIG. 6) implemented in the DC/ACconverter.

Referring to FIG. 14, the DC/DC converter 6 may also be implemented as abuck converter. This buck converter includes a series circuit with aninductive storage element 64, such as a choke, and a switch 65 betweenthe first input terminal 21 and the first output terminal 61. Afreewheeling element 66, such as a diode, is connected between thesecond output terminal 62 and a circuit node common to the inductivestorage element 64 and the switch 65. A capacitive storage element 63,such as a capacitor, is connected between the input terminals 21, 22.

Like in the boost converter of FIG. 12, the switch 65 in the buckconverter of FIG. 14 can be implemented as a conventional electronicswitch, such as a MOSFET or an IGBT, or could be implemented as aGaN-HEMT. Further, the freewheeling element 66 could be implemented as asynchronous rectifier.

Like in the boost converter according to FIG. 12, the switch 65 in thebuck converter according to FIG. 14 is driven by a PWM drive signal S65provided by a control circuit 67. The control circuit 67 may beimplemented as illustrated in FIG. 13. The operating principle of thecontrol circuit 67 in the buck converter of FIG. 14 is the same as inthe boost converter of FIG. 12, i.e., the duty cycle of the drive signalS65 is increased when the input voltage V3 is to be decreased, and theduty cycle is decreased, when the input voltage V3 is to be increased.

It should be noted that implementing the DC/DC converter 6 as a boostconverter (see FIG. 12) or as a buck converter (see FIG. 14) is only anexample. The DC/DC converter 6 could also be implemented as a buck-boostconverter, a boost-buck-converter, a flyback converter, and so on.Whether a boost converter or a buck converter is used as a DC/DCconverter for tracking the maximum power point of the DC source 3 andfor providing the input voltage V6 to the DC/AC converter 4, influencesthe number of converter units 2 to be connected in series in order forthe sum of the output voltages v2 of the converter units 2 to correspondto the external AC voltage v1. This will be explained by the way of anexample in the following.

Assume that there is an external AC voltage v1 with 240V_(RMS) isdesired. The peak voltage (maximum amplitude) of this voltage v1 is 338V(240V·sqrt(2), where sqrt is the square root). Further assume that theDC sources 3 are PV arrays each providing an output voltage between 24Vand 28V when exposed to sunlight. The DC/AC converter 4 has a buckcharacteristic, which means that the peak value of the output voltage v2(see FIG. 4) is less than the received DC input voltage V3 or V6,respectively. Thus, when buck converters are employed as DC/DCconverters 6 in the converter units 2 or when no DC/DC converters areused, at least 15 converter units 2 with PV panels connected theretoneed to be connected in series. This is based on the assumption thateach PV array generates a minimum voltage of V3=24V and that a peakvoltage of the external AC voltage v1 is 338V. The number of 15 isobtained by simply dividing 338V through 24V (338V/24V=14.08) androunding the result to the next higher integer.

When, however, a boost converter is used as the DC/DC converter 6 that,for example, generates an output voltage V6=60V from the input voltageV3 (which is between 24V and 28V) the number of converter units 2 to beconnected in series may be reduced to about 6.

In the DC/AC converter illustrated in FIG. 11, the output voltage V6 ofthe DC/DC converter may vary dependent on the input power received atthe input terminals 21,22 from the DC source 3 and dependent on theoutput current i1 or, more exactly, dependent on the average of theoutput current i1. According to a further embodiment illustrated in FIG.15, the control circuit 5 is further configured to control the inputvoltage of the DC/AC converter 4 and the output voltage of the DC/DCconverter 6, respectively. For this, the control circuit 5 receives aninput voltage signal SV6 that represents the input voltage V6. Thecontrol circuit 5 is configured to adjust the input voltage V6 byvarying the duty cycle of those switches in the DC/AC converter 4 thatare driven in a clocked fashion. The input voltage can be increased bygenerally decreasing the duty cycle and can be decreased by generallyincreasing the duty cycle. For this, the control circuit 5 includes afurther control loop, where this control loop is slower than the controlloop that causes the output current i1 to follow the reference signalS_(REF). This control loop is, for example configured to causevariations of the duty cycle at a frequency of between 1 Hz and 10 Hz.

The control circuit 5 of FIG. 15 is based on the control circuitillustrated in FIG. 8 and additionally includes a further control loopthat serves to adjust the amplitude of the output current referencesignal S_(i1-REF) dependent on the input voltage signal S_(V6). Insteadof the control loop illustrated in FIG. 8, the control circuit accordingto FIG. 15 could also be implemented based on the control circuit ofFIG. 10. Referring to FIG. 15, the control loop includes: a furthersubtraction element 56, a filter 55, and a multiplier 57. Thesubtraction element 56 receives the input voltage signal S_(V6) and areference signal S_(V6-REF) that represents a set value of the inputvoltage V6. The subtraction element 56 generates a further error signalbased on a difference between the input voltage signal S_(V6) and thereference signal S_(V6-REF). The filter 55 receives the further errorsignal and generates an amplitude signal S_(AMPL) representing anamplitude of the reference signal S_(REF) from the further error signal.The filter may have a P-characteristic, an I-characteristic, aPI-characteristic, or a PID-characteristic. The amplitude signalS_(AMPL) and the output signal of the VCO 52 are received by themultiplier 57 that provides the output current reference signalS_(i1-REF). The output current reference signal S_(i1-REF) has anamplitude that is dependent on the input voltage V6 and that serves tocontrol the input voltage V6 of the DC/AC converter (4 in FIG. 11), anda frequency and phase of the output current i1. The frequency and thephase of the reference signal S_(REF) are dependent on the at least onesynchronization signal S_(v1) and the output current signal S_(i1) andserve to adjust frequency and phase of the output current i1 such thatthere is a given phase difference between the output current and theoutput voltage.

The input voltage reference signal S_(V6-REF) may have a fixed valuethat is, selected such that the input voltage V6 is sufficiently belowthe voltage blocking capability of switches employed in the DC/ACconverter. However, it is also possible to vary the input voltagereference signal S_(V6-REF) dependent on the output current,specifically on the rms value of the output current i1. According to oneembodiment, the input voltage reference signal S_(V6-REF) decreases whenthe output current i1 increases, and the input voltage reference signalS_(V6-REF) increases when the output current decreases. According to oneembodiment, the input voltage reference signal S_(V6-REF) has a firstsignal value when the output current i1 is below a given thresholdvalue, and has a lower second signal value when the output current i1 isabove a given threshold value.

The control circuit illustrated in FIG. 15 could also be implemented ina converter as illustrated in FIG. 6 in which the DC/DC converter isomitted. In this case, the input voltage to be controlled is the outputvoltage V3 of the PV module, so that the voltage signal S_(V6) in FIG.15 is replaced by the voltage signal S_(V3) representing the outputvoltage of the DC source 3, and the input voltage reference signalS_(V6-REF) is replaced by the reference signal S_(V3-REF) defining adesired output voltage of the DC source 3. The input voltage referencesignal S_(V3-REF) may in this case be provided by an MPPT in order tooperate the DC source (PV module) 3 in its MPP.

FIG. 16 illustrates a further embodiment of DC/DC converter 6 that canbe implemented in a DC/AC converter unit 2 of FIG. 11. The DC/DCconverter of FIG. 16 is implemented as a boost converter with twoconverter stages 60 ₁, 60 ₂. The two converter stages 60 ₁, 60 ₂ areconnected in parallel between the input terminals 21, 22 and the outputterminals 61, 62. Each of the converter units 60 ₁, 60 ₂ is implementedlike the boost converter of FIG. 12 and includes a series circuit withan inductive storage element 64 ₁, 64 ₂, such as a choke, and a switch65 ₁, 65 ₂ between the input terminals of the DC/DC converter 6, wherethe input terminals of the DC/DC converter 6 correspond to the inputterminals 21, 22 of the converter unit 2. Further, each converter stageincludes a rectifier element 66 ₁, 66 ₂ such as a diode, connectedbetween a circuit node common to the corresponding inductive storageelement 64 ₁, 64 ₂ and the corresponding switch 65 ₁, 65 ₂ and the firstoutput terminal 61 of the DC/DC converter 6. The second output terminal62 of the DC/DC converter 6 is connected to the second input terminal22.

The two converter stages 60 ₁, 60 ₂ share the first capacitive storageelement 63 between the input terminals 21, 22, and share the secondcapacitive storage element 68 between the output terminals 61, 62. Theoutput voltage V6 of the DC/DC converter 6 is available across thesecond capacitive storage element 68.

Referring to FIG. 16, the control circuit (controller) 67 of the DC/DCconverter 6 generates two PWM drive signals S65 ₁, S65 ₂, namely a firstdrive signal S65 ₁ for the switch 65 ₁ of the first converter stage 60₁, and a second drive signal S65 ₂ for the switch 65 ₂ of the secondconverter stage 60 ₂. According to one embodiment, the first and secondboost converter stages 60 ₁, 60 ₂ are operated interleaved, which meansthat there is a time offset between the switching cycles of the firstswitch 65 ₁ and the switching cycles of the second switch 65 ₂.Providing two converter stages 60 ₁, 60 ₂ and operating these converterstages 60 ₁, 60 ₂ in an interleaved mode helps to reduce voltage ripplesof the input voltage V3 and the output voltage V6 of the DC/DC converter6. Of course, more than two boost converter stages 60 ₁, 60 ₂ can beconnected in parallel.

Referring to FIG. 16, each boost converter stage 60 ₁, 60 ₂ provides anoutput current I6 ₁, I6 ₂. These output currents I6 ₁, _(I62) add andform the overall output current I6 of the DC/DC converter. FIG. 17illustrates a first embodiment of the controller 67 configured togenerate PWM drive signals S65 ₁, S65 ₂ for each converter stage 60 ₁,60 ₂, and further configured to generate the PWM drive signals S65 ₁,S65 ₂ such that the output currents I6 ₁, I6 ₂ of the converter stages60 ₁, 60 ₂ are balanced.

Referring to FIG. 17 the control circuit 67 is based on the controlcircuit 67 of FIG. 13 and includes the subtraction element 671 receivingthe input voltage signal S_(V3) and the input voltage reference signalS_(REF-V3) and the filter 672 for providing the duty cycle signalS_(DC). The controller 67 of FIG. 17 further includes a first PWM driver673 ₁ receiving a first duty cycle signal S_(DC1) that is dependent onthe duty cycle signal S_(DC) provided by the filter 672 and receiving afirst clock signal CLK₁, and a second PWM driver 673 ₁ receiving asecond duty-cycle signal S_(DC2) that is dependent on the duty cyclesignal S_(DC) provided by the filter 672 and receiving a second clocksignal CLK₂. According to one embodiment, the first and second clocksignals CLK₁, CLK₂ have the same frequency. However, there is a phaseshift between the first and second clock signal CLK₁, CLK₂, so thatthere is a phase shift between the first PWM drive signal S65 ₁ providedby the first PWM driver 673 ₁ and the second PWM drive signal S65 ₂provided by the second PWM driver 673 ₂.

If the first and second converter stages 60 ₁, 60 ₂ would perfectlymatch so that there would be no risk of unbalanced output currents I6 ₁,I6 ₂, the duty cycle signal S_(DC) could be used as the first duty cyclesignal S_(DC1) and as the second duty cycle signal S_(DC2). However, dueto an inevitable mismatch of the components in the converter stages 60₁, 60 ₂ the output currents I6 ₁, I6 ₂ can be unbalanced when the firstand second drive signal S65 ₁, S65 ₂ would be generated with exactly thesame duty cycle.

In order to compensate for such unbalances of the first and secondoutput currents I6 ₁, I6 ₂, the controller 67 of FIG. 17 includes anadditional control loop, that can be referred to as current balancingloop or power balancing loop. This control loop receives a first outputcurrent signal S_(I61) representing the first output current I6 ₁ of thefirst converter stage 60 ₁, and a second output current signal S_(I62)representing the output current I6 ₂ of the second converter stage 60 ₂.These output current signals S_(I61), S_(I62) can be generated usingconventional current measurement units. The output current signalsS_(I61), m S_(I62) are received by a subtraction unit 675 that generatesa further error signal S_(ERR2). The further error signal S_(ERR2) isrepresentative of a difference between the first and second outputcurrents I6 ₁, I6 ₂. Further error signal S_(ERR2) is received by afilter 676 that generates a filtered error signal. The filter 676 mayhave a P-characteristic, a I-characteristic, or a PI-characteristic.

A further subtraction unit 674 ₁ subtracts the filtered error signalfrom the duty cycle signal S_(DC) to generate the first duty cyclesignal S_(DC1), and an adder 674 ₂ adds the filtered error signal to theduty cycle signal DC to generate the second duty cycle signal S_(DC2).

The operating principle of the controller 67 of FIG. 17 is as follows.When the first and second output currents I6 ₁, I6 ₂ are identical, thefurther error signal S_(ERR2) is zero. In this case, the first dutycycle signal S_(DC1) corresponds to the second duty cycle signalS_(DC2). When, for example, the first output current I6 ₁ is larger thanthe second output current I6 ₂, the further error S_(ERR2) and thefiltered error signal have a positive value. In this case, the dutycycle signal S_(DC1) (obtained by subtracting the filtered error signalfrom the duty cycle signal SDC) becomes smaller than the second dutycycle signal S_(DC2) (obtained by adding the filtered error signal tothe duty cycle signal S_(DC)). Thus, the duty cycle of the first drivesignal S65 ₁ becomes smaller than the duty cycle of the second drivesignal S65 ₂ in order to reduce the first output current I6 ₁ and toincrease the second output current I6 ₂, so as to balance these outputcurrents I6 ₁, I6 ₂.

FIG. 18 illustrates a further embodiment of the control circuit 67 thatis configured to balance the output currents I6 ₁, I6 ₂. The controlcircuit 67 of FIG. 18 is based on the control circuit 67 of FIG. 17. Inthe control circuit 67 of FIG. 18, the subtraction unit 674 ₁ thatgenerates the first duty cycle signal SDC1 does not receive the dutycycle signal SDC but receives a filtered version of a difference betweenthe duty cycle signal S_(DC) and the first output current signalS_(I61). A subtraction unit 677 ₁ calculates the difference and a filter678 ₁, filters the difference. The filter may have a P-characteristic,an I-characteristic or PI-characteristic. Equivalently, the adder 674 ₂that provides the second duty cycle signal S_(DC2) does not receive theduty cycle signal S_(DC) but receives a filtered difference between theduty cycle signal S_(DC) and the second input current signal S_(I62). Asubtraction unit 677 ₂ calculates the difference between the duty cyclesignal S_(DC) and the second output current signal S_(I62), and a filter678 ₂ filters the difference. The output signals of the filter 678 ₁,678 ₂ are received by the subtraction unit 674 ₁ and the adder 674 ₂,respectively.

While in the embodiment illustrated in FIG. 17 a single control loop isemployed to regulate the input voltage V3, a dual control loop structureis employed in the embodiment according to FIG. 18.

FIG. 19 illustrates a further embodiment of a converter unit 2 with aDC/AC converter 4. The converter unit 2 may further include a DC/DCconverter 6 (see FIG. 9) connected between the input terminals 21, 22and the DC/AC converter. However, such DC/DC converter is notillustrated in FIG. 13. Dependent on whether or not the converter unit 2includes a DC/DC converter the DC/AC converter 4 receives the inputvoltage V3 of the converter unit 2 or the output voltage of the DC/DCconverter 4 (not illustrated in FIG. 19) as an input voltage. Just forexplanation purposes it is assumed that the DC/AC converter 4 receivesthe input voltage V3.

The DC/AC converter of FIG. 19 includes a buck converter 80 thatreceives the input voltage V3 as an input voltage. The buck converter 80is configured to generate an output current i80 which is a rectifiedversion of the output current i1 of the DC/AC converter 4. Assume, forexample that a desired waveform of the output current i1 is a sinusoidalwaveform. In this case, the output current i80 provided by the converter80 has the waveform of a rectified sinusoidal curve or the waveform ofthe absolute value of a sinusoidal curve, respectively. This isschematically illustrated in FIG. 20, in which exemplary timing diagramsof a sinusoidal output current i1 and the corresponding output currenti80 of the converter 80 are illustrated.

The output current i1 of the DC/AC converter 4 is produced from theoutput current i80 of the buck converter 80 using a bridge circuit 85with two half-bridges, where each of these half-bridges is connectedbetween output terminals 81, 82 of the buck converter 80. This bridgecircuit 85 can be referred to as unfolding bridge. A first half-bridgeincludes a first and a second switch 85 ₁, 85 ₂ connected in seriesbetween the output terminals 81, 82, and a second half-bridge includes athird switch 85 ₃ and a fourth switch 85 ₄ connected in series betweenthe output terminals 81, 82. An output terminal of the firsthalf-bridge, which is a circuit node common to the first and secondswitches 85 ₁, 85 ₂ is coupled to the first output terminal 23. Anoutput terminal of the second half-bridge, which is a circuit nodecommon to the third and fourth switch 85 ₃, 85 ₄ is coupled to thesecond output terminal 24 of the converter unit 2. Optionally, an EMIfilter 88 with two inductances, such as chokes, is coupled between theoutput terminals of the half-bridges and the output terminals 23, 24 ofthe converter unit 2. The output capacitance C of the converter unit 2that is connected between the output terminals can be part of the EMIfilter 88.

Referring to FIG. 19, the output current i80 of the buck converter 80has a frequency which is twice the frequency of the output current i1. Aswitching frequency of the switches 85 ₁-85 ₄ of the bridge circuit 85corresponds to the frequency of the output current i1. During a positivehalf-cycle of the output current i1 the first and fourth switch 85 ₁, 85₄ are switched on, and during a negative half-cycle of the outputvoltage v2 the second and third switches 85 ₂, 85 ₃ are switched on. Theswitches of the bridge circuit 85 are driven by drive signals S85 ₁-S85₄ generated by a drive circuit 88. Timing diagrams of these drivesignals S85 ₁-S85 ₄ are also illustrated in FIG. 20. In FIG. 20, a highsignal level of these timing diagrams represents an on-level of thecorresponding drive signal S85 ₁-S85 ₄. An on-level of the drive signalis a signal level at which the corresponding switch is switched on. Thedrive signals S85 ₁-S85 ₄ may, for example, be generated dependent onthe output voltage v80 of the buck converter 80, where, according to oneembodiment, drive circuit 88 changes the switching state of the switcheseach time the output voltage v80 has decreased to 0. “Changing theswitching state” means either switching the first and the fourthswitches 85 ₁, 85 ₄ on and the other two switches off, or meansswitching the second and the third switch 85 ₂, 85 ₃ on and the othertwo switches off.

The buck converter 8 may have a conventional buck converter topology andmay include a switch 83 connected in series with an inductive storageelement 84, where the series circuit is connected between the firstinput terminal 21 of the converter unit 2 or the first output terminal61 of a DC/DC converter (not shown), and the first output terminal 81 ofthe buck converter 80, respectively. A rectifier element 86 is connectedbetween the second output terminal 82 (corresponding to the second inputterminal 22) of the buck converter and a circuit node common to theswitch 83 and the inductive storage element 84. The switch 83 can beimplemented as a conventional electronic switch, such as a MOSFET or anIGBT, or as a GaN-HEMT. The rectifier element 86 can be implemented as adiode or as a synchronous rectifier. Further, a capacitive storageelement 85, such as a capacitor, is connected between the inputterminals of the buck converter 80, and an optional smoothing capacitor89 is connected between the output terminals 81, 82.

The switch 83 of the buck converter 80 is driven by a PWM drive signalS83 generated by a control circuit or controller 87. The controller 87of the buck converter 80 receives the reference signal S_(REF) from thecontroller 5 of the converter unit 2. The controller 87 of the buckconverter 80 is configured to generate its output current i80 incorrespondence with the reference signal S_(REF). This reference signalS_(REF) according to FIG. 19, unlike the reference signal S_(REF) ofFIG. 11, does not have the waveform of the output current i1, but hasthe waveform of the rectified output current i1. This reference signalS_(REF) is also generated from the synchronization signal S_(v1) and theoutput current signal S_(i1).

The controller 5 for generating the reference signal S_(REF) accordingto FIG. 19 may correspond to the controllers illustrated in FIGS. 8 and15 with the difference that the oscillating signal provided at theoutput of the oscillator 53 is rectified. An embodiment of thecontroller 5 according to FIG. 19 is illustrated in FIG. 21. Thiscontroller 5 corresponds to the controller according to FIG. 8 with thedifference that the output signal of the filter 53 is received by arectifier 58 that generates a rectified version of the oscillatingoutput signal of the oscillator 53. Mathematically this is equivalent toforming the absolute value of the oscillating output signal of theoscillator 53. The reference signal S_(REF) is available at the outputof the rectifier 58.

FIG. 22 illustrates a further embodiment of a controller 5 that canimplemented in the DC/AC converter 4 of FIG. 19. The controller 5 ofFIG. 22 is based on the controller 5 of FIG. 15 with the difference thatthe amplitude signal S_(AMPL) is generated from the input voltage signalS_(V3) voltage signal S_(V3) that represents the input voltage V3provided by the DC source 3, and from the input voltage reference signalS_(REF-V3). The input voltage reference signal S_(REF-V3) can begenerated by an MPPT, such as an MPPT 7 explained with reference to FIG.11.

The control loops illustrated in FIGS. 15, 21 and 22 could, of course,be amended to be based on the control loop structure of FIG. 10 insteadof FIG. 8.

Referring to FIG. 19, the controller 87 of the buck converters 80 can beimplemented like a conventional controller for providing a PWM drivesignal in a buck converter. The controller 86 receives the referencesignal S_(REF) and an output current signal Si80, where the outputcurrent signal S_(i80) represents the output current vi80 of the buckconverter 80. The controller 86 is configured to vary the duty cycle ofthe drive signal S83 such that the output current i80 of the buckconverter 80 is in correspondence with the reference signal S_(REF). Thefunctionality of this controller 86 corresponds to the functionality ofthe controller 67 illustrated in FIG. 13. In the embodiment illustratedin FIG. 19 the controller receives the output current signal S_(i1)representing the output current i1 and the synchronization signal S_(v1)for generating the reference signal S_(REF). However, this is only anexample. It would also be possible to generate the reference signalS_(REF) based on signals representing the output voltage v80 and theoutput current i80 of the buck converter 80. In this case, the referencesignal is generated such that output current i80 and the output voltagev80 of the buck converter 80 have a given phase difference.

The operating principle of a power converter circuit 1 including DC/ACconverters as illustrated in FIG. 19 will now be explained withreference to FIGS. 1 and 19. The explanation will be based on theassumption that the voltage of the power grid 100 is a sinusoidalvoltage so that an output current i1 with a sinusoidal waveform isdesired. Further, it is assumed that the input powers of the individualDC/AC converters is zero, while the power grid voltage v_(N) is appliedto the input terminals 11, 12 and the bridge circuits 85 in theindividual converter units are in operation. In this case, the smoothingcapacitors 89 of the buck converters are connected in series between theoutput terminals 11, 12. When the individual capacitors 89 have the samesize, the voltage across each of these capacitors 89 is 1/n times thepower grid voltage v_(N).

Assume now that the DC/AC converters receives an input power from the PVmodules 3 connected thereto. The DC/AC converters then adjust theircommon output current i1 to be in phase with the external voltage v1(the power grid voltage). The amplitude of the output current i1 is, inparticular, controlled through the input voltage V3, where the currentis increased when the voltage V3 increases, and the current is decreasedwhen the voltage V3 decreases.

When the output current i1 provided by one DC/AC converter decreases, acurrent that corresponds to a difference between the output current i1and the common current i1OUT is provided by the output capacitor C whichcauses the voltage v2 across the output capacitor C to decrease untilthe input power provided to the DC/AC converter corresponds to itsoutput power. A decrease of the voltage v2 across the output capacitor89 of one DC/AC converter 4 or one converter unit 2 causes an increaseof the voltages across the output capacitors of the other converterunits. This process proceeds until the converter unit 2 has settled instable operation point at a lower output current i1. If the otherconverter units 2 at first continue to run at the same duty cycle, theincrease of the voltages across their output capacitors leads to areduction of their output currents i1 (and hence to a reduction of thecommon output current) in order to keep their output powers equal theirinput powers. When the output current i1 provided by one DC/AC converterincreases so as to be higher than the common current i1 _(OUT), thecorresponding output capacitor C is charged which results in an increaseof the voltage across the output capacitor C of the one converter and adecrease of the voltage across the output capacitors of the otherconverters.

It became obvious from the explanation provided before that besides thecontrol loops in the individual converter units 2 no additional controlloop is required in order to control the output voltages of theindividual converter units 2. The power converter circuit 1 with theconverter units 2 is “self organizing”. Referring to FIG. 1, assumethat, for example, in the steady state the input power provided by thefirst DC source 3 ₁ to the first converter unit 2 ₁ would drop, forexample because the corresponding PV array is shaded. The output voltagev2 ₁ of the corresponding converter unit 2 would then drop, while theoutput voltages of the other converter units 2 ₂, 2 _(n) would increasein order to meet the condition defined by equation (1). Further, thecommon output current i1 _(OUT) would decrease. The transient process isas follows: When the input power received by the first converter unit 2₁ decreases, the common output current i1 _(OUT) at first remainsunchanged, while the output current i1 ₁ of the first converter unit 2 ₁decreases. The decrease of the output current i11 and the unchangedcommon output current i_(OUT1) causes a discharging of the outputcapacitor C₁ of the first converter unit 2 ₁ so that the output voltagev2 ₁ decreases. A decrease of the output voltage of the first converterunit, however, causes an increase of the output voltages of the otherconverters, which now decrease their output currents in order to keeptheir output powers equal their input powers. The transition processesfinishes when a “new” common output current i_(OUT) has settled in towhich the individual output currents i1 correspond. This is aself-organizing and self-stabilizing process that does not require andadditional control loop besides the control loops in the individualconverter units 2 disclosed before.

FIG. 23 illustrates a further embodiment of a power converter circuit.In this power converter circuit two series circuits 1_(I), 1 _(II) eachincluding one group with a plurality of converter units 2 _(II)-2 _(In)and 2 _(III)-2 _(IIn) connected in series are connected in parallelbetween the output terminals 11, 12. Each of the series circuits 1 _(I),1 _(II) can be implemented in accordance with the series circuit 1 ofconverter units 2 ₁-2 _(n) explained before. The converter units of thetwo groups (the two series circuits) are coupled to the samesynchronization circuit 10 that can be implemented in accordance withone of the embodiments explained before. Of course, more than two seriescircuits each with a plurality of converter units can be connected inparallel.

Referring to the explanation herein before, the synchronization circuit10 can be implemented as a voltage measurement circuit that measures theexternal AC voltage v1 and that generates the at least onesynchronization signal S_(v1) such that the synchronization signal is acontinuous signal representing the external AC voltage v1 and,therefore, having the same frequency and phase as the external ACvoltage v1. FIG. 24 illustrates a further embodiment of thesynchronization circuit 10.

In the embodiment illustrated in FIG. 24, the synchronization circuit 10receives the external AC voltage v₁ available at the output terminals11, 12 and generates the synchronization signal S_(v1) as a continuoussignal with a frequency and a phase that is dependent on the frequencyand the phase of the external AC voltage v₁, respectively. According toone embodiment, the synchronization circuit 10 receives a phase-shiftsignal SPS that defines a desired phase shift between thesynchronization signal S_(v1) and the external AC voltage v₁. In theembodiment illustrated in FIG. 24, the synchronization circuit 10includes a phase-shift circuit 110 that provides an output voltage v₁′.The output voltage v₁′ of the phase-shift circuit 110 has a phase shiftrelative to the external AC voltage v₁, with the phase shift beingdefined by the phase-shift signal S_(PS). A transmission circuit 120receives the output voltage v₁′ of the phase-shift circuit 110 andgenerates the at least one synchronization signal S_(v1) transmitted tothe individual converter units 2 (not illustrated in FIG. 24).

Referring to FIG. 25, the transmission circuit 120 may be implemented asa voltage divider having a plurality of voltage divider elements 120 ₁,120 ₂, 120 _(n) connected in series. The voltage divider of thetransmission circuit 120 is similar to the voltage divider illustratedin FIG. 3. The individual voltage divider elements 120 ₁-120 _(n) of thevoltage divider 120 can be implemented like the voltage divider elements10 _(i) explained with reference to FIGS. 4A to 4C and 5. Referring toFIG. 25, each of the voltage divider elements 120 ₁-120 _(n) provides avoltage v1 _(1′), v1 _(2′), v1 _(n′), with each of these voltagesrepresenting one synchronization signal received by one converter unit 2(not illustrated in FIG. 25) in the same way as the individual converterunits 2 ₁-2 _(n) of FIG. 3 receive the individual voltages v1 ₁, v1 ₂,v1 _(n).

When employing a synchronization circuit 10 as illustrated in FIG. 24, aphase shift between the output currents i1 of the individual converterunits 2 and the external AC voltage v1 can be adjusted through thephase-shift signal S_(PS), so that there is no need to individuallyprovide phase shift signals (S_(φ) in FIGS. 15, 21 and 22) to theindividual converter units 2. However, it is of course possible toadditionally provide phase shift signals to the individual converterunits 2.

Transmitting the at least one synchronization signal S_(v1) to theindividual converter units through a voltage divider, as illustrated inFIG. 25, is only one possible embodiment. According to furtherembodiments, the at least one synchronization signal S_(v1) istransmitted to the individual converter units 2 via a signal bus, aradio path, or via a power line using power line communication. Ofcourse, corresponding receiver circuits are employed in the converterunits 2 in this case.

Referring to power line communication, standard power line communicationcircuits can be used for the communication between the synchronizationcircuit 10 and the individual converter units 2 since the output currentBecause i_(OUT) of the converter circuit is an AC current. In this case,the power line of the converter circuit 1, which is the line carryingthe output current i_(OUT) and connecting the outputs of the individualconverter units 2 is used for the communication. A first power linecommunication interface (not shown) coupled to the power line receivesthe synchronization signal S_(v1) and forwards the synchronizationsignal S_(v1) suitably modulated via the power line to the individualconverter units 2. Each converter unit includes a corresponding powerline communication interface coupled to the power line and configured toreceive and demodulate the modulated synchronization signal S_(v1).

According to a further embodiment, that is illustrated in dashed linesin FIG. 24, the synchronization circuit does not only receive aphase-shift signal S_(PS) but additionally to the phase shift signalS_(PS) or instead of the phase shift signal S_(PS) receives a controlsignal S_(CTRL) through which other parameters of the synchronizationsignal S_(V1), such as the frequency and/or the amplitude of thesynchronization signal S_(V1) can be adjusted. In this embodiment, thesynchronization signal S_(V1) can be generated independent of theexternal AC voltage v₁, which may be helpful in some operation scenariosexplained below.

Referring to the explanation herein before, the synchronization signalS_(v1) received by each converter unit 2 can be a continuous signal thatis continuously transmitted from the synchronization circuit 10 to theconverter units 2. Each of the converter units 2 continuously generatesits corresponding output current i1 in accordance with thesynchronization signal S_(v1), which means with a frequency and a phasedefined by the synchronization signal S_(v1).

According to a further embodiment, the synchronization signal S_(v1) isa pulse signal including a sequence of signal pulses and the individualconverter 2 units are configured to generate a continuous signal havinga frequency and a phase from the pulse signal.

FIG. 26 illustrates an embodiment of a converter unit 2 that isconfigured to receive a pulse signal as a synchronization signal S_(v1).The converter unit of FIG. 26 corresponds to the converter units ofFIGS. 5 and 11 and additionally includes a signal generator 20 thatreceives the pulsed synchronization signal S_(v1) and that is configuredto generate a continuous sinusoidal synchronization signal S_(v1′) fromthe pulsed signal S_(v1). In this embodiment, the continuoussynchronization signal S_(v1′) provided at the output of the signalgenerator 20 is received by the control circuit 5 and is processed inthe control circuit 5 in the same way as a continuous sinusoidalsynchronization signal S_(v1) explained herein before.

According to one embodiment, the pulsed synchronization signal S_(v1) isa periodic signal having signal pulses that are equally distant, andsignal generator 20 is configured to generate a sinusoidal signalS_(v1′) from the pulsed signal S_(v1). According to one embodiment, thesignal generator 20 generates the sinusoidal synchronization signalS_(v1′) such that a zero crossing of the sinusoidal signal S_(v1′)occurs each time a signal pulse of the pulsed synchronization signalS_(v1) occurs. In this embodiment, the mutual distance of the signalpulses defines the frequency of the continuous synchronization signalS_(v1′) generated by the signal generator 20 and the absolute positionof the individual signal pulses on the time scale defines the phase ofthe continuous synchronization signal. An embodiment of a signalgenerator 20 configured to receive a pulsed synchronization signalS_(v1) and configured to generate a continuous sinusoidalsynchronization signal S_(v1′) with a frequency and a phase as definedby the pulsed synchronization signal S_(v1) is illustrated in FIG. 27.Timing diagrams of signals occurring in this signal generator areillustrated in FIG. 28.

Referring to FIG. 27, the signal generator includes an integrator 202receiving the pulsed synchronization signal S_(v1) as a clock signal. Atiming diagram of an embodiment of the pulsed synchronization signalS_(v1) is illustrated in FIG. 28. The integrator is configured tointegrate a constant signal C received at a second input beginning witheach signal pulse of the pulsed synchronization signal S_(v1′). Anoutput signal of the integrator 202 is a ramp signal S_(RAMP) with afrequency corresponding to the frequency of the pulsed synchronizationsignal S_(v1′). The constant signal is provided by a calculation unit201 that receives pulsed signal S_(v1′) and calculates the constantsignal C to be proportional to the frequency of the pulsedsynchronization signal S_(v1′) or to be inversely proportional to a timeperiod T (see FIG. 28) of the pulsed signal S_(v1′). In the steadystate, the slope of the individual ramps of the ramp signal S_(RAMP) isdependent on the frequency (and decreases when the frequency is reduced)and the amplitudes of the individual ramps are equal. According to oneembodiment, the calculation unit 201 calculates the constant value C ineach cycle of the pulsed signal S_(v1′) and provides the calculatedvalue to the integrator in the next cycle. Thus, a frequency change ofthe synchronization becomes effective in the generation of the rampsignal S_(RAMP) with a delay of one cycle of the pulsed signal.

Referring to FIG. 27, a trigonometric function generator 203 receivesthe ramp signal S_(RAMP) and generates the continuous synchronizationsignal S_(v1′) by calculating the sine or the cosine of instantaneousvalues of the ramp signal S_(RAMP). The resulting continuoussynchronization signal S_(v1′) is illustrated in FIG. 28. In theembodiment illustrated in FIGS. 27 and 28 the continuous synchronizationsignal S_(v1′) has a zero crossing from negative to positive signalvalues each time a signal pulse of the pulsed synchronization signaloccurs.

Of course, the signal generator of FIG. 27 could easily be modified togenerate the continuous synchronization signal S_(v1′) such that witheach pulse of the pulsed signal S_(v1) a zero crossing from positive tonegative signal values occurs.

According to one embodiment, the pulsed synchronization signal S_(v1) isonly transmitted for a short time when a frequency and/or phase of thepulsed signal changes. This means, only a short sequence with somecycles of the pulsed signal S_(v1) is transmitted, while after thetransmission of the sequence the pulsed signal is interrupted for a timeperiod significantly longer than one cycle period. This interruption maybe several seconds or several minutes. In this embodiment, a clockgenerator receives the pulsed signal S_(v1). The clock generator isconfigured to measure the frequency of the pulsed signal S_(v1) and togenerate a clock signal provided to the integrator with a frequencycorresponding to the measured frequency of the pulsed signal S_(v1). Theclock generator is, in particular configured to store the frequencyinformation and to generate the clock signal even in those time periodswhen the pulsed signal S_(v1) has been switched off and updates thefrequency each time a new sequence of the pulsed signal S_(v1) istransmitted. Equivalently, the calculation unit stores the calculatedvalue C until a new sequence of the pulsed signal S_(v1) is transmittedthat allows the calculation unit 201 to re-calculate the constant value.

According to a further embodiment, the synchronization signal S_(v1) isan AC signal that is only transmitted for a certain time period, suchas, for example, for a duration corresponding to only several periods ofthe AC signal. In this embodiment, the signal generator 20 is configuredto evaluate a frequency and a phase information of the synchronizationsignal S_(v1) and is configured to generate the continuoussynchronization signal S_(v1) based on this frequency and timeinformation. In this embodiment, the synchronization signal S_(v1) maybe transmitted only once at the beginning of the operation of the powerconverter circuit 1 to the individual converter units, or may betransmitted periodically during the operation of the power convertercircuit 1.

An embodiment of a signal generator 20 that is configured to generate acontinuous (sinusoidal) synchronization signal S_(v1′) from asynchronization signal S_(v1) that is available for only several periodsis illustrated in FIG. 29. The signal generator of FIG. 29 is based onthe signal generator of FIG. 27 and additionally includes a zerocrossing detector 205 receiving the synchronization signal S_(v1) andconfigured to generate a pulse signal. The pulse signal generated by thezero crossing detector includes a signal pulse each time a positive or anegative zero crossing is detected. The pulse signal provided by thezero crossing detector 205 is then processed by the clock generator 204,the calculation unit 201, the integrator 202, and the trigonometricfunction generator 203 as explained with reference to FIGS. 27 and 28.In this embodiment, the continuous synchronization signal S_(v1′) issynchronized to the synchronization signal S_(v1′) during that timeperiod when the synchronization signal S_(v1) is available and, afterthe synchronization signal has been switched off, continues to generatethe continuous synchronization signal S_(v1′) based on the frequency andphase information stored in the clock generator 204 and the calculationunit 201.

According to a further embodiment, the synchronization signal S_(v1)transmitted to the individual converter units corresponds to thefrequency and phase signal S_(ωt) explained with reference to FIGS. 15,21 and 22. In this embodiment, the signal generator 20 can be omittedand the control circuit 5 can be simplified by omitting the PLL 51.

In each of the individual cases where different signal waveforms of thesynchronization signal S_(v1) have been discussed, the synchronizationsignal S_(v1) can be generated by the synchronization circuit 10connected between output terminals 11, 12.

So far, the operation of the power converter circuit in a normaloperation mode has been explained. In the normal operation mode, each ofthe individual converter units 2 is configured to generate its outputcurrent i1 such that the output current i1 has a frequency and the phaseas defined by the synchronization signal Sv1 received by the converterunit 2. Besides the normal operation mode other operation modes of thepower converter circuit 1 as well.

According to one embodiment, that is schematically illustrated in FIG.30, the power converter circuit 1 is either operated in the normal mode901 or in a standby mode 902. In the standby mode 902 the individualconverter units 2 are deactivated so that the output currents i1 of theindividual converter units 2 are zero, but may again be activated.

For example, the power converter is, e.g. in the standby mode, when thesupply voltages (V3 ₁-V3 _(n) in FIG. 1) provided by the DC powersources are too low for generating the output currents i1. When theindividual DC power sources 3 ₁-3 _(n) are implemented as PV modules,this may occur at night.

The power converter circuit 1 switches from the normal mode 901 to thestandby mode 902 when a shut-down condition is met, and changes from thestandby mode 902 to the normal mode when a start-up condition is met.The process of switching the power converter circuit 1 from the normalmode to the standby mode will be referred to as shut down and a sequenceof operations involved in this process will be referred to as shut-downsequence in the following. The process of switching the power convertercircuit 1 from the standby mode to the normal mode will be referred toas start-up and a sequence of operations involved in this process willbe referred to as start-up sequence in the following.

The power converter circuit 1 may include an operation mode controllerthat defines the operation mode of the power converter circuit 1. Withother words, the operation mode controller 50 controls the overalloperation of the power converter circuit 1. FIG. 31 illustrates a blockdiagram of a power converter circuit 1 that includes an operation modecontroller 50. The operation mode controller 50 can be implemented as amicroprocessor, an ASIC, a digital signal processor, a state machine, orthe like.

In the embodiment illustrated in FIG. 31, the operation mode controller50 receives at least one operation parameter of the power convertercircuit 1 from a measurement unit 60, is configured to control aconnection circuit 70 connected between the series circuit with theindividual converter units 2 ₁-2 _(n) and the output terminals 11, 12,and is configured to control the synchronization unit 10. Themeasurement circuit 60 is configured to measure at least one of theoutput current i_(OUT) of the series circuit with the converter units 2₁-2 _(n), and a voltage v_(OUT) across the series circuit 2 ₁-2 _(n). Asschematically illustrated in FIG. 31, the measurement circuit 60 mayinclude a current measurement circuit 60 for measuring the outputcurrent i_(OUT), and a voltage measurement circuit 602 for measuring theoutput voltage v_(OUT). The output voltage v_(OUT) across the seriescircuit corresponds to the external AC voltage v₁ when the seriescircuit is connected to the output terminals 11, 12. The connectioncircuit 70 that is configured to either connect the series circuit 2 ₁-2_(n) to the output terminals 11, 12 or to disconnect the series circuit2 ₁-2 _(n) from the output terminals 11, 12 may include two switches,namely a first switch 701 connected between the series circuit 2 ₁-2_(n) and the first output terminal 11 and a second switch 702 connectedbetween the series circuit 2 ₁-2 _(n)and the second output terminal 12.These switches 701, 702 can be implemented as conventional switches,such as relays or semiconductor switches (MOSFETs, IGBTs, etc.).Referring to FIG. 31, the connection circuit 70 may include an optionalthird switch 703 connected in parallel with the series circuit 2 ₁-2_(n). This switch 703 may be closed when an output voltage of the seriescircuit with the individual converter units 2 is above a given voltagethreshold, in order to limit the output voltage. Optionally, a resistoror another type of current limiting element is connected in series withthis switch 703.

In FIG. 31, signal S60 provided by the measurement circuit 60 to theoperation mode controller 50 represents the at least one operationparameter measured by the measurement circuit 60. This measurementsignal S60 includes information on at least one of the output currenti_(OUT) and the output voltage v_(OUT). Signal S70 in FIG. 31schematically illustrates a control signal generated by the operationmode controller 50 and received by the connection circuit 70. Dependenton the control signal S70, the connection circuit 70 connects the seriescircuit to the output terminals 11, 12, disconnect the series circuitfrom the output terminals 11, 12, or short-circuits the series circuit 2₁-2 _(n).

Referring to FIG. 31, the operation mode controller 50 further controlsthe synchronization circuit 10 that generates the synchronization signalS_(v1). In FIG. 31, only the control signal S_(CTRL) is drawn to bereceived by the synchronization circuit 10. The control signal S_(CTRL)defines the signal parameters of the synchronization signal S_(v1), suchas frequency, phase and amplitude. As controlled by the control signalS_(CTRL) the synchronization signal S_(v1) can dependent on the externalAC voltage v₁ also received by the synchronization circuit 10, such ashave a given phase shift (zero or other than zero) relative to theexternal AC voltage, or the synchronization signal S_(v1) can beindependent of the external AC voltage v₁. Referring to the explanationbelow, there may be operating scenarios (such as fault ride through)where it is necessary to generate the synchronization signal S_(v1)independent of the external AC voltage v₁.

In the power converter circuit of FIG. 31, the synchronization signalS_(v1) is not only used in the normal mode to provide a synchronizationinformation to the individual converter units 2 for generating theoutput currents i1, but is also used in the standby mode for signallingthe individual converter units 2 that a change from the standby mode tothe normal mode is desired. In this power converter circuit 1, theoperation mode controller 50 has the synchronization circuit 10 generatethe synchronization signal S_(v1) with a standby waveform in the standbymode. The standby waveform is a waveform that is different from thesignal waveform of the synchronization signal S_(v1) in the normal mode.According to one embodiment, the standby waveform is a waveform with aconstant signal value, such as zero.

FIG. 32 illustrates an embodiment of a converter unit 2 that isconfigured to evaluate the operation mode information included in thesynchronization signal S_(v1) and that can be operated in a normal modeor a standby mode. The overall power converter circuit 1 is in thenormal mode when each of the converter units 2 is in the normal mode andis in the standby mode when each of the converter units is in thestandby mode 2. The converter unit 2 shown in FIG. 32 is based on theconverter units of FIGS. 5, 11 and 26, where the DC/DC converter 6 andits control circuit 7, and the signal generator 20 are optional. Theconverter unit 2 includes an operation mode unit 30 that receives thesynchronization signal S_(v1) and that is configured to evaluate thesynchronization signal S_(v1). The operation mode unit 30 is, inparticular, configured to detect a change of the synchronization signalS_(v1) from the standby waveform to the normal waveform, the latterbeing the usual waveform in the normal mode. Referring to theexplanation above, the normal waveform can be a continuous AC waveform,a pulsed signal waveform, or an AC waveform with only periods.

The operation mode unit 30 is further configured to control the DC/ACconverter 4, in particular to activate the DC/AC converter 4 in thenormal mode and to deactivate the DC/AC converter in the standby mode.When the converter unit 2 further includes the DC/DC converter 6, theoperation mode unit 30 further controls the operation (activates ordeactivates) of the DC/DC converter 6. When the DC/AC converter 4 andthe optional DC/DC converter 6 is activated, the operating principle ofthe converter unit 2 corresponds to the operating principle explainedbefore, which means the converter unit 2 provides an output current i1in accordance with the synchronizations signal S_(v1). When the DC/ACconverter 4 and the optional DC/DC converter 6 are deactivated, theswitches (see FIGS. 6 and 19) in the DC/AC converter 4 and the DC/DCconverter are either switched off, or some of the switches arepermanently switched on. This is explained in greater detail below.

In the standby mode, the operation mode controller 50 either disconnectsthe series circuit 2 ₁-2 _(n) from the output terminals 11, 12 and,therefore, from the external AC voltage v₁, or leaves the series circuit2 ₁-2 _(n) connected to the output terminals 11, 12.

Embodiments of start-up sequences for switching from the standby mode tothe normal mode are explained below. For explanation purposes it isassumed that the individual DC power sources are PV modules. In thiscase, a start-up sequences is required at least once a day, namely inthe morning after sunrise.

A first embodiment of a start-up sequence (start-up sequence A) isillustrated in FIG. 33. In this embodiment, the individual converterunits 2 in the standby mode 902 are configured to pass the input voltageV3 from the input terminals 21, 22 through to the output terminals 23,24, and the operation mode controller 50 is configured to have theconnection circuit 70 disconnect the series circuit 2 ₁-2 _(n) from theoutput terminals 11, 12.

The input voltage V3 can be connected through the converter unit 2 tothe output terminals 23, 24 by switching on switches in the DC/ACconverter 4 and the optional DC/DC converter 6 in a specificconfiguration. When, for example, the DC/AC converter 4 is implementedwith a H4-bridge as illustrated in FIG. 6, the input voltage V3 can beconnected through to the output terminals 23, 24 by permanentlyswitching on the first switch 421 and the fourth switch 424. When theoptional DC/DC converter 6 is a boost converter, as illustrated in FIG.12, the switch 65 is permanently switched off, and when the optionalDC/DC converter 6 is a buck converter, as illustrated in FIG. 14, theswitch 65 is permanently switched on. The switching states of theswitches in the DC/AC converter 4 and the DC/DC converter 6 in thestandby mode is governed by the operation mode unit 30.

When, for example, the DC/AC converter is implemented with a buckconverter and an unfolding bridge as illustrated in FIG. 19, the inputvoltage V3 can be connected through to the output terminals 23, 24 bypermanently switching on the first switch 85 ₁ and the fourth switch 85₄ in the unfolding bridge 85 and by switching on the switch 83 in thebuck converter 80.

After sunrise, the input voltage V3 at the input terminals 21, 22 and,therefore, the output voltage v2, which at this stage is a DC voltage,increases. The operation mode controller 50 is configured to detect theoutput voltage v_(OUT). The output voltage v_(OUT) is the sum of theoutput voltages v2 of the individual converter units 2, where thisoutput voltage v_(OUT) increases after sunrise when the solar powerreceived by the PV modules increases. When the output voltage v_(OUT)reaches a given threshold voltage v_(OUT-TH), the operation modecontroller 50 controls the synchronization circuit 10 to generate thesynchronization signal S_(v1) with the normal waveform has theconnection circuit 70 connect the series circuit 2 ₁-2 _(n) to theoutput terminals 11, 12. Referring to the explanation before, thesynchronization signal S_(v1) in a normal mode can be a continuous ACsignal, a periodic pulse signal, or an AC signal for a limited timeduration.

The operation mode unit 30 detects the change of the synchronizationsignal S_(v1) from the standby level to the normal level. The operationmode unit 30 then activates the DC/AC converter 4 and the optional DC/DCconverter 6 to operate as explained with reference to FIGS. 1 to 23before. According to one embodiment, the DC/AC converter 4 and theoptional DC/DC converter 6 are activated at the time of a zero crossingof the synchronization signal S_(v1), so as to ramp up the outputcurrent i1.

According to one embodiment, not only frequency and phase of the outputcurrent i1, but also the amplitude of the output current i1 iscontrolled during the start-up phase so as to, e.g., continuouslyincrease the output current in the start-up phase. The output current i1of each converter 2 can be controlled by controlling the input power ofthe converter 2. Controlling the input power is possible in eachconverter topology in which the input voltage V3 is controlled, that isin each topology where the input voltage V3 is adjusted dependent on aninput voltage reference signal S_(V3-REF). In the normal mode, the inputvoltage reference signal S_(V3-REF) may be generated by an MPP tracker(see, circuit block 7 in FIGS. 11 and 32) that serves to operate PVmodules 3 providing the input voltage V3 in an optimum operation point.In order to control the input voltage V3 and, therefore, in order tocontrol the output current i1 during start-up, the operation modecontrol circuit 30 can be configured to provide the input voltagereference signal S_(REF-V3) during start-up or can be configured tocontrol the MPP tracker 7 during start-up. This is schematicallyillustrated in dotted lines in FIG. 32. During the start-up phase, thePV modules 3 are not necessarily operated in their MPP. According to oneembodiment, the operation mode control circuit 30 increases the inputvoltage reference signal S_(REF-V3) stepwise in two, three or moresteps, so as to stepwise increase the amplitude of the AC output currenti1 of the individual converter units 2.

When in the converter unit 2 of FIG. 32, the DC/AC converter 4 includesa buck converter 80 and an H4-bridge 85, as illustrated in FIG. 19, thebuck converter 80 can be configured to control the input voltage V3. TheDC/DC converter 6 may be omitted in this case. An embodiment of acontrol circuit 5 that is configured to control the input voltage V3 inthe DC/AC converter 4 of FIG. 19 is illustrated in FIG. 22. While in thenormal mode, the input voltage reference signal S_(V3-REF) is providedby an MPP tracker (not illustrated in FIGS. 19 and 22), the inputvoltage reference signal S_(V3-REF) may be provided by the operationmode unit 30 during the start-up phase in order to control the outputcurrent i1 during the start-up phase.

Switching on the switches in the DC/AC converter 4 and the optionalDC/DC converter 6 in the standby mode requires a power supply. Referringto FIG. 32, each converter unit 2 includes a power supply unit 40 thatprovides for the power supply of the individual components in theconverter unit 2. The power supply unit 40 is either connected to theinput terminals 21, 22, to the output terminals 23, 24 or, when there isa DC link capacitor between the DC/DC converter 6 and the DC/ACconverter 4, to the DC link capacitor.

When the power supply unit 40 is connected to the input terminals 21,22, energy for switching on the switches in the DC/AC converter 4 andthe DC/DC converter 6 is, of course, only provided when an input voltageV3 other than zero is provided by the DC power source. Thus, aftersunrise, the input voltage V3 first powers the power supply unit 40,which powers the components in the power converter unit 2, which thenpasses the input voltage V3 through to the output terminals 23, 24, theoutput voltage v2 is then detected by the operation mode controller 50,which then has the converter unit 2 change to the normal mode by havingthe synchronization circuit 10 change the synchronization signal S_(v1)from the standby waveform to the normal waveform. Before solar power isprovided to the PV modules, i.e. when the input voltage V3 is zero, eachof the switches in the converter unit 2 is switched off and theconverter unit cannot be activated. This operation mode can be referredto as shut-off mode.

Start-Up Sequence B

A second embodiment of a start-up sequence (start-up sequence B) isillustrated in FIG. 34. In this embodiment, the operation modecontroller 50 leaves the series circuit 2 ₁-2 _(n) connected to theoutput terminals 11, 12 when the power converter circuit 1 is in thestandby mode. The individual converter units 2 are deactivated, so thatthe output current i_(OUT) is zero and the output voltage v_(OUT)corresponds to the external AC voltage v₁. The external AC voltage v₁charges the input capacitor of the DC/AC converter 4, which is the DClink capacitor when a DC/DC converter 6 and a DC/AC converter 4 areemployed. The charging of the input capacitor of the DC/AC converter 4is explained for the DC/AC converter topologies of FIGS. 6 and 19 below.Referring to FIG. 6, the switches of the H4-bridge each have afreewheeling element 42 ₁-42 ₄. Via these freewheeling elements theinput capacitor 41 (or the DC link capacitor 60 of FIG. 11) is chargedto the peak value of the AC voltage v2 between the output terminals 23,24, when the switches 42 ₁-42 ₄ are switched off. Thus, in the standbymode, the operation mode unit (30 in FIG. 32) controls the switches of aDC/AC converter 4 implemented with a H4-bridge to be switched off.

When the DC/AC converter 4 is implemented with an unfolding bridge 85 asillustrated in FIG. 19, the DC link capacitor 89 is charged throughfreewheeling elements (not illustrated in FIG. 19) of the individualswitched 85 ₁-85 ₄ to the peak value of the AC input voltage v2.

In this embodiment, the power supply unit 40 is connected to the inputcapacitor of the DC/AC converter 4 or to the DC link capacitor whichpermanently provides for a power supply of the converter unit 2.

While in the start-up sequence A the power converter automaticallyenters the normal mode when sufficiently high input voltages V3 areprovided, an additional trigger signal is required in the start-upsequence B informing the operation mode controller 50 that the powerconverter circuit 1 may switch from the standby mode to the normaloperation mode. According to one embodiment, the trigger signal is asignal indicating the sunrise and, therefore the time when enough solarpower is expected to be received by the individual PV modules in orderto successfully switch from the standby mode to the normal mode. Thistrigger signal can be provided from an external source to the operationmode controller 50 or can be calculated in the operation mode controller50 dependent on the specific date, the geographical position of the PVmodules and a table that includes the time of sunrise at thegeographical position at different dates. This signal triggering aswitching from the standby mode to the normal mode will be referred toas trigger signal or sunrise signal in the following.

Start-Up Sequence C

According to a further embodiment (start-up sequence C), which includesfeatures from both, the start-up sequences A and B, the operation modecontroller 50 leaves the series circuit 2 ₁-2 _(n) disconnected from theoutput terminals 11, 12 in the standby mode. Further, the converterunits 2 are configured to pass through the input voltage V3 to theoutput terminals 23, 24 in the standby mode. In this embodiment,switching of the power converter circuit 1 from the standby mode to thenormal mode is initiated by the sunrise signal. Again, switching fromthe standby mode to the normal mode includes changing the waveform ofthe synchronization signal S_(v1) from the standby waveform to thenormal waveform.

There may be several reasons for the power converter circuit 1 to switchfrom the normal mode to the standby mode. According to one embodiment,the operation mode controller 50 is also configured to cause the powerconverter circuit 1 to switch from the normal mode to the standby modewhen the operation mode controller 50 detects the occurrence of ashut-down condition. A shut-down information can be transmitted from theoperation mode controller 50 to the individual converter units 2 indifferent ways. When a shut down information is received by theindividual converter units 2, the converter units are deactivated andenter the standby mode.

As explained above in connection with start-up sequence I, the operationmode controller 50 can be configured to only start-up the powerconverter circuit 1 when the output voltage vOUT in the standby mode ishigher than a given reference voltage. When the output voltage vOUT istoo low, this may have several reasons: First, the solar power receivedby the PV modules can be too low. Second, there are not enough converterunits 2 connected in series.

Transmission of Shut-Down Information I

According to a first embodiment, the synchronization signal S_(v1) isused to transmit a shut-down information from the operation modecontroller 50 to the individual converter units 2. Independent of thewaveform of the synchronization signal S_(v1) in the normal mode, theoperation mode controller 50 simply controls the synchronization circuit10 to generate a standby waveform of the synchronization signal S_(v1).The operation mode units 30 in the individual converter 2 are configuredto detect the standby waveform and to deactivate the correspondingconverter unit upon detection of the standby waveform. In the standbymode, the output currents i1 of the individual converter units 2 becomezero.

Transmission of Shut-Down Information II

According to a further embodiment, the operation mode controller 50 hasthe connection circuit 70 to disconnect the series circuit from theoutput terminals 11, 12 when a switching from the normal mode to theshut-off mode is desired. When the series circuit 2 ₁-2 _(n) isdisconnected from the power grid and when the converter units 2 arestill in the normal mode, the output current provided by each converterunit 2 causes the output voltages v2 of the individual converter units 2to increase, so that the overall output voltage v_(OUT) increases. Inthis embodiment, the converter units 2, are configured to detect theiroutput voltage v2 and are configured to enter the standby mode when theoutput voltage increases to an overvoltage threshold. According to oneembodiment, the operation mode unit 30 of each converter unit 2 monitorsthe output voltage v2 and compares the output voltage with theovervoltage threshold and shuts down the converter unit 2 when theoutput voltage v2 reaches the overvoltage threshold. According to oneembodiment, the overvoltage threshold is chosen to be dependent on thevoltage blocking capability of the semiconductor switches employed inthe DC/AC converter 4 of each converter unit 2.

In this embodiment, there is no direct transmission of information fromthe operation mode controller 50 to the individual converter units 2.Instead, the switching information is provided by allowing the outputvoltages v2 of the individual converter units 2 to increase to theovervoltage threshold.

Also in those cases in which the synchronization signal is used totransmit the switching information, so that there is no intendedovervoltage in the individual converter units 2, an overvoltage of theoutput voltages of one converter unit 2 may occur, e.g., whendisconnecting the series circuit 2 ₁-2 _(n) from the power grid. Thus,an overvoltage protection may be implemented in the individual converterunits 2 in each case,

Some embodiments of shut-down conditions (errors) that can be detectedby the operation mode controller are explained below. Dependent on thetype of error, the operation mode controller 50 may try to restart thepower converter circuit 1 after a certain time, or may keep the powerconverter circuit shut down.

Low Output Current

According to one embodiment, the power converter circuit switches fromthe normal mode to the standby mode, when the output current falls belowa given current threshold. This transfer is initiated by the operationmode controller 50 that compares the output current iOUT based oninformation received from the measurement unit 60 with the currentthreshold. The current threshold is, for example, chosen from a range ofbetween 0.2 A and 0.5 A.

Undervoltage Condition

Another type of error may occur when the solar power received by each ofthe converter units 2 is low. In this case, the output current i_(OUT)of the series circuit with the individual converter units 2 may have anon-sinusoidal waveform such that the waveform of the output currenti_(OUT) follows the waveform of the external AC voltage v₁ when theinstantaneous value of the output voltage v1 is low, and that the outputcurrent i_(OUT) is kept on a constant value or even decreases at higherinstantaneous values of the output current. This type of error can bedetected by the operation mode controller 50 by comparing the waveformof the output voltage v_(OUT) or the external AC voltage v₁,respectively, and the output current i_(OUT). When this type of error isdetected by the operation mode controller 50, the operation modecontroller 50 initiates one of the shut-down sequences explained abovein order to switch the power converter circuit 1 into the standby mode.

Phase Difference

According to a further embodiment, the operation mode controller 50 isconfigured to measure a phase difference between a phase of the externalAC voltage v₁ and the output current i_(OUT). When this phase differenceis larger than a desired phase difference, namely the phase differencegiven by the synchronization signal S_(v1) v₁ and/or the phasedifference as defined by the phase signals S_(φ), two different coursesof action initiated by the operation mode controller 50 are possible.When, for example, the phase difference between the output voltagei_(OUT) and the external AC voltage v₁ is below a first phase differencethreshold, the phase difference of the synchronization signal S_(v1)relative to the external AC voltage v₁ can be changed in order toreadjust the phase difference between the output current i_(OUT) and theexternal AC voltage v₁. When, however, the phase difference is above thephase-difference threshold, the operation mode controller 50 may shutdown the power converter circuit 1 using one of the shut-down sequencesexplained before.

Sunset

Similar to having the power converter start-up using a trigger signal atsunrise, a corresponding trigger signal can be used to shut-down thepower converter circuit at sunset.

Automatic Shut Down

When, for example, the solar power received by some of the PV modules ismuch lower than the solar power received by other modules, the outputvoltage of the converter units 2 connected to the PV modules receiving alow solar power decreases, while the output voltage of the otherconverter units 2 increases. This mechanism has been explained in detailherein before. When there are several PV modules that receive asignificantly lower solar power than other modules, the external ACvoltage v₁ applied to the output terminals 11, 12 may result in anovervoltage at the outputs of the other converter units 2. The converterunits 2 having an overvoltage may shut down, which results in anovervoltage at the outputs of other converter units 2, which are thenshut down. This proceeds, until each of the converter units 2 is shutdown. When the converter units 2 are shut down, the output currentbecomes zero. In this case, the individual converter units 2automatically shut down, so that no shut-down information has to betransmitted from the operation mode controller 50 to the individualconverter units 2. A decrease of the output current to zero is detectedby the operation mode controller 50 which may then cause thesynchronization circuit 10 to generate a standby waveform of thesynchronization signal S_(v1).

The operation mode controller 50 may not only be configured to monitorthe operation of the power converter circuit 1, but may also beconfigured to monitor the power grid, specifically the external ACvoltage v₁, in order to shut down the power converter circuit 1 when anerror is detected.

Anti-Islanding

A first type of grid error that may occur is “islanding”. In this case,the power grid has a high input impedance at the input terminals 7, 12.This error can be detected by having the series circuit with theconverter units 2 generate a constant output current i_(OUT) or an ACoutput current i_(OUT) with a frequency that is different from thefrequency of the external AC voltage v₁. As explained hereinbefore, thefrequency of the output current i_(OUT) (which is zero, when the outputcurrent i_(OUT) is constant) can be adjusted through the synchronizationsignal S_(v1).

In order to test for the occurrence of an islanding error, the operationmode controller 50 can be configured to have the synchronization circuit10 generate the synchronization signal with a frequency other than thefrequency of the external AC voltage v₁. In a test mode in which theoperation mode controller 50 changes the output current i_(OUT) asexplained before, the operation mode controller 50 compares the waveformof the output current i_(OUT) with the waveform of the external voltagev₁ available at the output terminals 11, 12. When the waveform of theexternal voltage v₁ follows the waveform of the output current i_(OUT),the power grid has a high input impedance (or has even been switched offdistant to the output terminals 11, 12). In this case, the operationmode controller shuts down the power converter circuit 1.

Interruption of Grid Voltage

According to one embodiment, the operation mode controller 50 isconfigured to monitor the external AC voltage v₁ and is configured toshut down the power converter circuit 1 when the external AC voltage v₁is switched off or interrupted.

Fault Ride Through

According to one embodiment, the operation mode controller 50 does notshut down the power converter circuit 1 immediately when the external ACvoltage v₁ is interrupted, but has the series circuit generate an ACoutput current i_(OUT) for a specified time period, such as, forexample, several milliseconds (ms). The operation mode controller 1shuts down the power converter circuit 1 when the external AC voltage v₁has not recovered after this specified time period. The operation modein which an AC output current i_(OUT) is provided although the externalAC voltage v₁ has been interrupted, is out phase, lower than usual,distorted, short-circuited, etc., is referred to as “fault ridethrough”.

In the fault-ride-through mode, the synchronization information inaccordance to which the individual converter units 2 generate theiroutput currents i1 can be provided in different ways. An embodiment, inwhich the synchronization information is only transmitted at thebeginning of the normal mode and in which a continuous synchronizationsignal is generated (in the signal generator 20) in the individualconverter units 2, no additional synchronization information needs to beprovided in the fault-ride-through mode. When, however, the individualconverter units 2 require a continuous synchronization signal, and whenthe synchronization signal in the normal mode is generated from theexternal AC voltage v₁, the synchronization circuit 10 in thefault-ride-through mode continuous to generate a continuoussynchronization signal based on the frequency and phase information ofthe synchronization signal generated before in the normal mode, i.e.before an interruption of the external AC voltage v₁ has been detected.

Reactive Power Generation

The power converter circuit 1 may even be used to stabilize the voltageon the power grid.

Referring to the explanation provided before, in the normal mode, theoutput current i_(OUT) generated by the series circuit of the individualconverter units 2 has a frequency and a phase as defined by thesynchronization signal S_(v1). The frequency and the phase of thesynchronization signal S_(v1) can be adjusted by the operation modecontroller 50. In the normal mode, the synchronization signal S_(v1) isusually generated such that the frequency information included in thesynchronization signal S_(v1) corresponds to the frequency of theexternal AC voltage v₁ and the phase information corresponds to thephase of the external AC voltage v₁. In this case, the output currenti_(OUT) is in phase with the external AC voltage v₁.

However, there may be situations in which it is desired to have a phasedifference between the output current i_(OUT) and the external ACvoltage v₁, in order to provide reactive power to the power grid so asto stabilize the voltage on the power grid. This phase difference caneasily be adjusted by suitably adjusting the phase information includedin the synchronization signal S_(v1). According to one embodiment, theoperation mode controller 50 receives an external signal from a utilityprovider, where this external signal includes a desired phase differencebetween the output current i_(OUT) and the external voltage v₁. Theexternal signal can be provided to the operation mode controller viaconventional communication channels, such as radio channels, powerlines, or the internet.

According to a further embodiment, the operation mode controller 50measures the output power provided by the power converter circuit 1 tothe power grid and adjusts the phase difference between the outputcurrent i_(OUT) and the external AC voltage v₁ dependent on the outputpower. According to one embodiment, the phase difference increases, soas to increase the reactive power provided to the net, when the outputpower provided by the power converter circuit 1 increases.

Active Power Derating

According to a further embodiment, the operation mode controller 50 isconfigured to detect the frequency of the external AC voltage and isconfigured to reduce the output power of the power converter circuit 1when the frequency reaches a frequency threshold such as 50.2 Hz or 60.3Hz that is above a set value, such as 50 Hz or 60 Hz. The frequency of agrid voltage may increase when there is more power input to the gridthan there is power consumed by consumers connected to the grid.

The output power of the power converter circuit 1 can be controlled bycontrolling the input voltages V3 of the individual converter units 2.This has been explained in connection with “start-up sequence A” before.The information that a reduction of the output power of the individualconverter units 2 is required, may be transmitted from the operationmode controller 50 to the individual converter units 2 through the samechannel through which the synchronization signal S_(v1) is transmitted.

Restart

Referring to the explanation above, there may be operation scenarioswhen the power converter circuit 1 is shut down after an error hasoccurred. After the power converter circuit 1 has been shut down thepower converter circuit can be restarted using one of the start-upsequences explained herein before. In the following, “to restart” thepower converter circuit 1 means to employ one of the start-up sequencesto again start the power converter circuit 1.

When, e.g., the power converter circuit 1 has been shut down due to anerror of the power grid, the operation mode controller 50 can beconfigured to check the external AC voltage v1 and can be configured torestart the power converter circuit 1 after the grid voltage v1 hasreturned to normal. The operation mode controller 50 may be configuredto check the grid voltage in regular time intervals, such as everyminute, every five minutes, etc.

When, e.g., the power converter circuit 1 has been shut down due to anundervoltage condition, due to automatic shutdown, or due to a phasedifference, the operation mode controller may be configured to restartthe power converter circuit after a given time period, such as, e.g.,one minute, two minutes, etc.

Of course, the occurrence of an error may also be detected duringstart-up so that it is even possible to shut down the power convertercircuit 1 before the normal operation mode has been reached.

Referring to explanation before, the output current i1 of the individualconverter units 2 may be increased in accordance with a given timeprofile during the start-up phase. This current profile may be fixedcurrent profile. According to a further embodiment, the profile of theoutput current i1 during start-up is limited dependent on the shut-downhistory, which means dependent on whether the power converter circuit 1has been shut down due to an error. According to one embodiment theoutput current is increased slower (in accordance with a shallowercurrent profile) when the power converter circuit 1 has been shut downdue to an undervoltage condition, due to automatic shutdown, or due to aphase difference. When the restart fails because an error has occurredduring the start-up phase, an even shallower current profile may beapplied after the next restart. A “shallower current profile” is aprofile in which the current increases slower.

In the embodiments explained before, the synchronization signal S_(v1)is provided by the synchronization circuit 10, where the synchronizationcircuit 10 is configured to generate the synchronization signal S_(v1)dependent on the external AC voltage v1, e.g., in the normal mode, orindependent of the external AC voltage, e.g., when an error hasoccurred.

According to a further embodiment illustrated in FIG. 35, thesynchronization circuit 10 includes synchronization units 10 ₁, 10 ₂, 10_(n) with each synchronization unit 10 ₁, ₁₀₂, 10 _(n) coupled to theoutput terminals of one converter unit 2 ₁, 2 ₂, 2 _(n), configured tomeasure the output voltage v2 ₁, v2 ₂, v2 _(n) of the correspondingconverter unit 2 ₁, 2 ₂, 2 _(n), to generate a synchronization signaldependent on the measured output voltage and to provide thesynchronization signal to the corresponding converter unit 2 ₁, 2 ₂, 2_(n). According to one embodiment, the individual synchronizationsignals are proportional to the output voltages v2 ₁, v2 ₂, v2 _(n), sothat the individual synchronization units 10 ₁, 10 ₂, 10 ₃ may beimplemented as voltage measurement units.

An embodiment of a converter unit 2 that can be used in the powerconverter circuit 1 of FIG. 35 is illustrated in FIG. 36. The converterunit 2 of FIG. 36 is based on the converter unit 2 explained in detailwith reference to FIG. 32. In the converter unit 2 of FIG. 36, thesynchronization signal S_(v1) is a voltage measurement signal receivedby measuring the output voltage v2 of the converter unit 2. Theoperating principle of the converter unit 2 of FIG. 36 is explainedbelow.

For explanation purposes it is assumed, that the power converter circuit1 is in the standby mode. In the standby mode, the power convertercircuit 1 is connected to the output terminals 11, 12 (see FIG. 35) sothat the external AC voltage v1 is applied to the series circuit withthe individual converter units 2. In the standby mode, when the outputpower of the power converter circuit 1 is zero, the output capacitances(C in the converter unit 2 of FIG. 36) of the individual converter units2 act as a capacitive voltage divider so that the voltages v2 at theoutputs of the individual converter units 2 are in phase with theexternal AC voltage v1. The start-up sequence employed to start up theindividual converter units 2 corresponds to start-up sequence Bexplained before, with the following differences:

At the beginning of the start-up sequence or before the beginning of thestart-up sequence, the synchronization signal S_(v1) is provided to thesignal generator 20 for a short time period, such as for several periodsof the synchronization signal S_(v1), which at this time is a sinusoidalsignal that is in phase with the external Ac voltage v1. The signalgenerator 20 synchronizes to the synchronization signal S_(v1) and thenautonomously generates the continuous synchronization signal S_(v1′) inthe start-up phase and in the normal mode after the start-up phase. Thesignal generator 20 can be implemented as explained with reference toFIG. 29 before.

Referring to FIG. 36, the operation mode unit 30 may control the timeperiod when the synchronization signal S_(v1) is provided to the signalgenerator 20. This is schematically illustrated by having a switch 301connected between the synchronization unit (not shown in FIG. 36) andthe signal generator 20, with the switch being controlled by theoperation mode unit. However, this serves to illustrate the operatingrather than the implementation. Of course, many different means may beemployed to provide the synchronization signal S_(v1) that is dependenton the output voltage v2 to the signal generator for a given time periodbefore or at the beginning of the start-up sequence.

In this converter circuit 1, after the converter circuit 1 has enteredthe normal mode, the operation mode controller 50 may be configured todetect a phase difference between the output current i1 and the externalAC voltage and to shut down the converter circuit 1 when the phasedifference exceeds a given threshold. The converter circuit 1 may beshut down as explained before in the paragraph TRANSMISSION OF SHUT-DOWNINFORMATION II. The restart mechanism may correspond to one of therestart mechanisms explained before. At a restart after the shutdown,the converter circuit 1 will again be synchronized to the externalvoltage v 1 as explained before.

According to a further embodiment, the operation mode controller 50provides a phase shift signal, corresponding to the phase shift signalS_(φ) explained before, to control circuits 5 of the individualconverter units 2. In this embodiment, the operation mode controller 50is configured to adapt the phase shift signal S_(φ) when the phasedifference between the output current i_(OUT) and the external voltagev1 is above a first phase difference threshold and below a second phasedifference threshold, in order to prevent a further a further increaseof the phase difference. Further, the operation mode controller 50 isconfigured to shut down the converter circuit in order to force arestart when a the phase difference is above the second phase differencethreshold.

Each of the circuits explained before may be implemented as analog ordigital circuit, or as a mixed circuit with analog and digital circuitmeans. Consequently, the signals explained before may be analog ordigital signals. In case of the synchronization signal S_(v1) orS_(v1′), respectively, “continuous synchronization signal” means thatthe synchronization signal is available in each period of the AC outputcurrent i1 and has a waveform corresponding to the waveform of thecorresponding output current i1.

Although various exemplary embodiments of the invention have beendisclosed, it will be apparent to those skilled in the art that variouschanges and modifications can be made which will achieve some of theadvantages of the invention without departing from the spirit and scopeof the invention. It will be obvious to those reasonably skilled in theart that other components performing the same functions may be suitablysubstituted. It should be mentioned that features explained withreference to a specific figure may be combined with features of otherfigures, even in those cases in which this has not explicitly beenmentioned. Further, the methods of the invention may be achieved ineither all software implementations, using the appropriate processorinstructions, or in hybrid implementations that utilize a combination ofhardware logic and software logic to achieve the same results. Suchmodifications to the inventive concept are intended to be covered by theappended claims.

Spatially relative terms such as “under,” “below,” “lower,” “over,”“upper,” and the like, are used for ease of description to explain thepositioning of one element relative to a second element. These terms areintended to encompass different orientations of the device in additionto different orientations than those depicted in the figures. Further,terms such as “first,” “second,” and the like, are also used to describevarious elements, regions, sections, etc., and are also not intended tobe limiting. Like terms refer to like elements throughout thedescription.

As used herein, the terms “having,” “containing,” “including,”“comprising,” and the like are open ended terms that indicate thepresence of stated elements or features, but do not preclude additionalelements or features. The articles “a” “an” and “the” are intended toinclude the plural as well as the singular, unless the context clearlyindicates otherwise.

With the above range of variations and applications in mind, it shouldbe understood that the present invention is not limited by the foregoingdescription, nor is it limited by the accompanying drawings. Instead,the present invention is limited only by the following claims and theirlegal equivalents.

It is to be understood that the features of the various embodimentsdescribed herein may be combined with each other, unless specificallynoted otherwise.

What is claimed is:
 1. A power converter circuit, comprising: outputterminals configured to receive an external voltage; and a seriescircuit with a plurality of converter units connected between the outputterminals of the power converter circuit, each converter unit comprisinginput terminals configured to be coupled to a DC power source and outputterminals configured to provide an output current, wherein at least oneconverter unit of the plurality of converter units comprises a signalgenerator configured to receive a synchronization signal and to generatea continuous synchronization signal from the synchronization signal;wherein the power converter circuit can be operated in a normaloperation mode, and wherein, in the normal operation mode, the at leastone converter unit is configured to regulate generation of the outputcurrent such that a frequency and/or a phase of the output current aredependent on the continuous synchronization signal.
 2. The powerconverter circuit of claim 1, wherein the synchronization signalcomprises an AC signal, and wherein the signal generator is configuredto receive the synchronization signal for a given time period, isconfigured to detect a frequency and a phase of the synchronizationsignal, and is configured to generate the continuous synchronizationsignal dependent on the detected frequency and phase after the giventime period.
 3. The power converter circuit of claim 2, wherein thesynchronization signal is dependent on a voltage between the outputterminals of the at least one converter unit.
 4. The power convertercircuit of claim 2, wherein the at least one converter unit isconfigured to generate the output current such that there is a givenphase difference between the output current and the continuoussynchronization signal.
 5. The power converter circuit of claim 1,wherein the synchronization signal is a pulsed signal comprising aplurality of signal pulses, wherein the signal generator is configuredto generate the continuous synchronization signal with a frequency and aphase dependent on the frequency and the phase of the pulsed signal. 6.The power converter circuit of claim 5, wherein the at least oneconverter unit is configured to generate the output current such thatthere is a given phase difference between the output current and thecontinuous synchronization signal.
 7. The power converter circuit ofclaim 5, further comprising a synchronization circuit configured toreceive the external voltage and to generate the synchronization signaldependent on the external voltage.
 8. The power converter of claim 7,wherein the synchronization circuit is configured to generate a signalpulse of the synchronization signal each time a positive or a negativezero crossing of the external voltage occurs.
 9. The power convertercircuit of claim 1, further comprising: an operation mode controller,configured to detect a shut-down condition and configured to interrupt aconnection between the series circuit and the output terminals upondetection of a shut-down condition.
 10. The power converter circuit ofclaim 9, wherein the operation mode controller is configured to detectthe shut-down condition when at least one of the following conditions ismet: an output current at the output terminals of the power convertercircuit is below a given current threshold; a phase difference betweenthe output current at the output terminals of the power convertercircuit and an output voltage at the output terminals of the powerconverter circuit is above a given phase difference threshold; or theoutput voltage falls below a given voltage threshold.
 11. The powerconverter circuit of claim 9, wherein the at least one converter unit isconfigured to generate zero output current when a voltage between theoutput terminals of the at least one converter unit reaches a givenvoltage threshold.
 12. The power converter circuit of claim 1, whereinthe at least one converter unit comprises: a DC/AC converter coupledbetween the input terminals and the output terminals of the at least oneconverter unit, the DC/AC converter configured to generate the outputcurrent with a frequency and phase that is dependent on a firstreference signal; and a control circuit configured to generate the firstreference signal dependent on the synchronization signal and the outputcurrent of the power converter unit.
 13. The power converter of claim12, wherein the DC/AC converter is configured to receive an inputvoltage, and wherein the control circuit is configured to generate thefirst reference signal dependent on the input voltage.
 14. The powerconverter circuit of claim 12, wherein the at least one converter unitfurther comprises: a DC/DC converter coupled between the input terminalsof the at least one converter unit and the DC/AC converter, the DC/DCconverter configured to adjust an input voltage between the inputterminals and/or an input current at the input terminals dependent on asecond reference signal; and a reference signal source configured toprovide the second reference signal.
 15. The power converter circuit ofclaim 14, wherein the reference signal source is implemented as amaximum power point tracker and is configured to generate the secondreference signal dependent on the input voltage and the input current ofthe at least one converter unit.
 16. The power converter of claim 14,wherein the DC/DC converter is a boost converter or a buck converter.17. The power converter circuit of claim 1, comprising: a plurality ofseries circuits connected in parallel, wherein the series circuit is oneof the plurality of series circuits and wherein each series circuitcomprises a plurality of converter units connected between the outputterminals, and wherein the converter units of the series circuits arecoupled to the synchronization circuit.
 18. A power supply system,comprising: output terminals configured to receive an external voltage;a series circuit with a plurality of converter units connected betweenthe output terminals of the power supply system, each converter unitcomprising input terminals and output terminals for providing an outputcurrent, wherein at least one converter unit comprises a signalgenerator configured to receive a synchronization signal and configuredto generate a continuous synchronization signal from the synchronizationsignal; and a plurality of DC power sources, each DC power sourcecoupled to the input terminals of one of the converter units, whereinthe power supply system can be operated in a normal operation mode, andwherein, in the normal operation mode, the at least one converter unitis configured to regulate generation of the output current such that afrequency and/or a phase of the output current are dependent on thecontinuous synchronization signal.
 19. The power supply system of claim18, wherein each DC power source comprises a photovoltaic array with atleast one solar cell.
 20. The power supply system, of claim 18, whereineach DC power source comprises a fuel cell.